ENH: Allow subplots axes to be passed into DataFrame.plot()
This works nicely:

```
Python 2.7.5+ (default, Sep 19 2013, 13:48:49) 
Type "copyright", "credits" or "license" for more information.

IPython 1.1.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: import pandas, numpy

In [2]: import matplotlib.pyplot as plt

In [3]: df1 = pandas.DataFrame(numpy.random.random((5, 4)))

In [4]: df2 = pandas.DataFrame(numpy.random.random((5, 4)))

In [5]: ax = df1.plot()

In [6]: df2.plot(ax=ax)
Out[6]: <matplotlib.axes.AxesSubplot at 0x3f4f910>

In [7]: plt.show()
```

But this doesn't:

```
In [8]: ax = df1.plot(subplots=True)

In [9]: df2.plot(subplots=True, ax=ax)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-9-70822353a5d1> in <module>()
----> 1 df2.plot(subplots=True, ax=ax)

/usr/local/lib/python2.7/dist-packages/pandas/tools/plotting.pyc in plot_frame(frame, x, y, subplots, sharex, sharey, use_index, figsize, grid, legend, rot, ax, style, title, xlim, ylim, logx, logy, xticks, yticks, kind, sort_columns, fontsize, secondary_y, **kwds)
   1634                      logy=logy, sort_columns=sort_columns,
   1635                      secondary_y=secondary_y, **kwds)
-> 1636     plot_obj.generate()
   1637     plot_obj.draw()
   1638     if subplots:

/usr/local/lib/python2.7/dist-packages/pandas/tools/plotting.pyc in generate(self)
    853         self._args_adjust()
    854         self._compute_plot_data()
--> 855         self._setup_subplots()
    856         self._make_plot()
    857         self._post_plot_logic()

/usr/local/lib/python2.7/dist-packages/pandas/tools/plotting.pyc in _setup_subplots(self)
    909                 ax = self._maybe_right_yaxis(ax)
    910             else:
--> 911                 fig = self.ax.get_figure()
    912                 if self.figsize is not None:
    913                     fig.set_size_inches(self.figsize)

AttributeError: 'numpy.ndarray' object has no attribute 'get_figure'
```

It would be nice if you could use the `subplots` flag, because it makes it easier to compare two data frames visually.
