BUG: boxplot fails when one column is all NaNs
Very much an edge case:

``` python
In [17]: df = pd.DataFrame(np.random.randn(100, 4))
In [18]: df.loc[:, 0] = np.nan
In [19]: df.plot(kind='box')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-19-06443674b77d> in <module>()
----> 1 df.plot(kind='box')

/Users/tom/Envs/py3/lib/python3.4/site-packages/pandas/pandas/tools/plotting.py 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, layout, **kwds)
   2362                              secondary_y=secondary_y, layout=layout, **kwds)
   2363 
-> 2364     plot_obj.generate()
   2365     plot_obj.draw()
   2366     return plot_obj.result

/Users/tom/Envs/py3/lib/python3.4/site-packages/pandas/pandas/tools/plotting.py in generate(self)
    913         self._compute_plot_data()
    914         self._setup_subplots()
--> 915         self._make_plot()
    916         self._add_table()
    917         self._make_legend()

/Users/tom/Envs/py3/lib/python3.4/site-packages/pandas/pandas/tools/plotting.py in _make_plot(self)
   2140             kwds = self.kwds.copy()
   2141 
-> 2142             ret, bp = plotf(ax, y, column_num=0, **kwds)
   2143             self.maybe_color_bp(bp)
   2144             self._return_obj = ret

/Users/tom/Envs/py3/lib/python3.4/site-packages/pandas/pandas/tools/plotting.py in plotf(ax, y, column_num, **kwds)
   2059             else:
   2060                 y = remove_na(y)
-> 2061             bp = ax.boxplot(y, **kwds)
   2062 
   2063             if self.return_type == 'dict':

/Users/tom/Envs/py3/lib/python3.4/site-packages/matplotlib-1.5.x-py3.4-macosx-10.9-x86_64.egg/matplotlib/axes/_axes.py in boxplot(self, x, notch, sym, vert, whis, positions, widths, patch_artist, bootstrap, usermedians, conf_intervals, meanline, showmeans, showcaps, showbox, showfliers, boxprops, labels, flierprops, medianprops, meanprops, capprops, whiskerprops, manage_xticks)
   3043         """
   3044         bxpstats = cbook.boxplot_stats(x, whis=whis, bootstrap=bootstrap,
-> 3045                                        labels=labels)
   3046         if flierprops is None:
   3047             flierprops = dict(sym=sym)

/Users/tom/Envs/py3/lib/python3.4/site-packages/matplotlib-1.5.x-py3.4-macosx-10.9-x86_64.egg/matplotlib/cbook.py in boxplot_stats(X, whis, bootstrap, labels)
   1963 
   1964         # medians and quartiles
-> 1965         q1, med, q3 = np.percentile(x, [25, 50, 75])
   1966 
   1967         # interquartile range

/Users/tom/Envs/py3/lib/python3.4/site-packages/numpy/lib/function_base.py in percentile(a, q, axis, out, overwrite_input)
   2818         axis = 0
   2819 
-> 2820     return _compute_qth_percentile(sorted, q, axis, out)
   2821 
   2822 # handle sequence of q's without calling sort multiple times

/Users/tom/Envs/py3/lib/python3.4/site-packages/numpy/lib/function_base.py in _compute_qth_percentile(sorted, q, axis, out)
   2824     if not isscalar(q):
   2825         p = [_compute_qth_percentile(sorted, qi, axis, None)
-> 2826              for qi in q]
   2827 
   2828         if out is not None:

/Users/tom/Envs/py3/lib/python3.4/site-packages/numpy/lib/function_base.py in <listcomp>(.0)
   2824     if not isscalar(q):
   2825         p = [_compute_qth_percentile(sorted, qi, axis, None)
-> 2826              for qi in q]
   2827 
   2828         if out is not None:

/Users/tom/Envs/py3/lib/python3.4/site-packages/numpy/lib/function_base.py in _compute_qth_percentile(sorted, q, axis, out)
   2854     # Use add.reduce in both cases to coerce data type as well as
   2855     #   check and use out array.
-> 2856     return add.reduce(sorted[indexer]*weights, axis=axis, out=out)/sumval
   2857 
   2858 def trapz(y, x=None, dx=1.0, axis=-1):

ValueError: operands could not be broadcast together with shapes (0,) (2,) 
```

Same result for `df.boxplot()`.
