Scatter plot should have a discrete colorbar when 'c' is integer
``` python
import pandas as pd
pd.set_option('max_rows', 10)
import sklearn.datasets as datasets
import pandas_ml as pdml  # https://github.com/pandas-ml/pandas-ml
import matplotlib.pyplot as plt
df = pdml.ModelFrame(datasets.load_iris())
print(df)
     .target  sepal length (cm)  sepal width (cm)  petal length (cm)  \
0          0                5.1               3.5                1.4
1          0                4.9               3.0                1.4
2          0                4.7               3.2                1.3
3          0                4.6               3.1                1.5
4          0                5.0               3.6                1.4
..       ...                ...               ...                ...
145        2                6.7               3.0                5.2
146        2                6.3               2.5                5.0
147        2                6.5               3.0                5.2
148        2                6.2               3.4                5.4
149        2                5.9               3.0                5.1

     petal width (cm)
0                 0.2
1                 0.2
2                 0.2
3                 0.2
4                 0.2
..                ...
145               2.3
146               1.9
147               2.0
148               2.3
149               1.8

[150 rows x 5 columns]

print(df.dtypes)
.target                int64
sepal length (cm)    float64
sepal width (cm)     float64
petal length (cm)    float64
petal width (cm)     float64
dtype: object
df.plot.scatter(x='sepal length (cm)', y='sepal width (cm)', c='.target')

plt.show()
```

![iris](https://cloud.githubusercontent.com/assets/109167/13136974/2a1ca272-d61e-11e5-9463-a7fa40fb28f2.png)
#### Expected Output

A scatter plot with a colobar with a discrete colormap
#### Additional information

Converting `.target` column to a category doesn't fix the problem.
Maybe I should open an other issue about it.

``` python
df['.target'] = df['.target'].astype('category')
df.plot.scatter(x='sepal length (cm)', y='sepal width (cm)', c='.target')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgb(self, arg)
    305                     else:
--> 306                         fl = float(argl)
    307                         if fl < 0 or fl > 1:

ValueError: could not convert string to float: '.target'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba(self, arg, alpha)
    369             else:
--> 370                 r, g, b = self.to_rgb(arg)
    371             if alpha is None:

//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgb(self, arg)
    327             raise ValueError(
--> 328                 'to_rgb: Invalid rgb arg "%s"\n%s' % (str(arg), exc))
    329             # Error messages could be improved by handling TypeError

ValueError: to_rgb: Invalid rgb arg ".target"
could not convert string to float: '.target'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba_array(self, c, alpha)
    398             # Single value? Put it in an array with a single row.
--> 399             return np.array([self.to_rgba(c, alpha)], dtype=np.float)
    400         except ValueError:

//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba(self, arg, alpha)
    375             raise ValueError(
--> 376                 'to_rgba: Invalid rgba arg "%s"\n%s' % (str(arg), exc))
    377

ValueError: to_rgba: Invalid rgba arg ".target"
to_rgb: Invalid rgb arg ".target"
could not convert string to float: '.target'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgb(self, arg)
    305                     else:
--> 306                         fl = float(argl)
    307                         if fl < 0 or fl > 1:

ValueError: could not convert string to float: '.'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba(self, arg, alpha)
    369             else:
--> 370                 r, g, b = self.to_rgb(arg)
    371             if alpha is None:

//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgb(self, arg)
    327             raise ValueError(
--> 328                 'to_rgb: Invalid rgb arg "%s"\n%s' % (str(arg), exc))
    329             # Error messages could be improved by handling TypeError

ValueError: to_rgb: Invalid rgb arg "."
could not convert string to float: '.'

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
<ipython-input-8-19145e105567> in <module>()
----> 1 df.plot.scatter(x='sepal length (cm)', y='sepal width (cm)', c='.target')

//anaconda/lib/python3.5/site-packages/pandas/tools/plotting.py in scatter(self, x, y, s, c, **kwds)
   3847         axes : matplotlib.AxesSubplot or np.array of them
   3848         """
-> 3849         return self(kind='scatter', x=x, y=y, c=c, s=s, **kwds)
   3850
   3851     def hexbin(self, x, y, C=None, reduce_C_function=None, gridsize=None,

//anaconda/lib/python3.5/site-packages/pandas/tools/plotting.py in __call__(self, x, y, kind, ax, subplots, sharex, sharey, layout, figsize, use_index, title, grid, legend, style, logx, logy, loglog, xticks, yticks, xlim, ylim, rot, fontsize, colormap, table, yerr, xerr, secondary_y, sort_columns, **kwds)
   3669                           fontsize=fontsize, colormap=colormap, table=table,
   3670                           yerr=yerr, xerr=xerr, secondary_y=secondary_y,
-> 3671                           sort_columns=sort_columns, **kwds)
   3672     __call__.__doc__ = plot_frame.__doc__
   3673

//anaconda/lib/python3.5/site-packages/pandas/tools/plotting.py in plot_frame(data, x, y, kind, ax, subplots, sharex, sharey, layout, figsize, use_index, title, grid, legend, style, logx, logy, loglog, xticks, yticks, xlim, ylim, rot, fontsize, colormap, table, yerr, xerr, secondary_y, sort_columns, **kwds)
   2554                  yerr=yerr, xerr=xerr,
   2555                  secondary_y=secondary_y, sort_columns=sort_columns,
-> 2556                  **kwds)
   2557
   2558

//anaconda/lib/python3.5/site-packages/pandas/tools/plotting.py in _plot(data, x, y, subplots, ax, kind, **kwds)
   2382         plot_obj = klass(data, subplots=subplots, ax=ax, kind=kind, **kwds)
   2383
-> 2384     plot_obj.generate()
   2385     plot_obj.draw()
   2386     return plot_obj.result

//anaconda/lib/python3.5/site-packages/pandas/tools/plotting.py in generate(self)
    985         self._compute_plot_data()
    986         self._setup_subplots()
--> 987         self._make_plot()
    988         self._add_table()
    989         self._make_legend()

//anaconda/lib/python3.5/site-packages/pandas/tools/plotting.py in _make_plot(self)
   1557             label = None
   1558         scatter = ax.scatter(data[x].values, data[y].values, c=c_values,
-> 1559                              label=label, cmap=cmap, **self.kwds)
   1560         if cb:
   1561             img = ax.collections[0]

//anaconda/lib/python3.5/site-packages/matplotlib/__init__.py in inner(ax, *args, **kwargs)
   1810                     warnings.warn(msg % (label_namer, func.__name__),
   1811                                   RuntimeWarning, stacklevel=2)
-> 1812             return func(ax, *args, **kwargs)
   1813         pre_doc = inner.__doc__
   1814         if pre_doc is None:

//anaconda/lib/python3.5/site-packages/matplotlib/axes/_axes.py in scatter(self, x, y, s, c, marker, cmap, norm, vmin, vmax, alpha, linewidths, verts, edgecolors, **kwargs)
   3891                 offsets=offsets,
   3892                 transOffset=kwargs.pop('transform', self.transData),
-> 3893                 alpha=alpha
   3894                 )
   3895         collection.set_transform(mtransforms.IdentityTransform())

//anaconda/lib/python3.5/site-packages/matplotlib/collections.py in __init__(self, paths, sizes, **kwargs)
    829         """
    830
--> 831         Collection.__init__(self, **kwargs)
    832         self.set_paths(paths)
    833         self.set_sizes(sizes)

//anaconda/lib/python3.5/site-packages/matplotlib/collections.py in __init__(self, edgecolors, facecolors, linewidths, linestyles, antialiaseds, offsets, transOffset, norm, cmap, pickradius, hatch, urls, offset_position, zorder, **kwargs)
    115
    116         self.set_edgecolor(edgecolors)
--> 117         self.set_facecolor(facecolors)
    118         self.set_linewidth(linewidths)
    119         self.set_linestyle(linestyles)

//anaconda/lib/python3.5/site-packages/matplotlib/collections.py in set_facecolor(self, c)
    610             c = mpl.rcParams['patch.facecolor']
    611         self._facecolors_original = c
--> 612         self._facecolors = mcolors.colorConverter.to_rgba_array(c, self._alpha)
    613         self.stale = True
    614

//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba_array(self, c, alpha)
    420             result = np.zeros((nc, 4), dtype=np.float)
    421             for i, cc in enumerate(c):
--> 422                 result[i] = self.to_rgba(cc, alpha)
    423             return result
    424

//anaconda/lib/python3.5/site-packages/matplotlib/colors.py in to_rgba(self, arg, alpha)
    374         except (TypeError, ValueError) as exc:
    375             raise ValueError(
--> 376                 'to_rgba: Invalid rgba arg "%s"\n%s' % (str(arg), exc))
    377
    378     def to_rgba_array(self, c, alpha=None):

ValueError: to_rgba: Invalid rgba arg "."
to_rgb: Invalid rgb arg "."
could not convert string to float: '.'

```
#### output of `pd.show_versions()`

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Darwin
OS-release: 14.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8

pandas: 0.17.1
nose: 1.3.7
pip: 8.0.2
setuptools: 19.6.2
Cython: 0.23.4
numpy: 1.10.4
scipy: 0.17.0
statsmodels: 0.6.1
IPython: 4.0.1
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.4.2
pytz: 2015.7
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.4.4
matplotlib: 1.5.1
openpyxl: 2.2.6
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.7.7
lxml: 3.4.4
bs4: 4.4.1
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.0.9
pymysql: None
psycopg2: None
Jinja2: 2.8
```

Pinging @sinhrks 

See also http://stanford.edu/~mwaskom/software/seaborn/examples/scatterplot_matrix.html
