Cannot update boxplot via axes
When I construct a boxplot with pandas, e.g. like this:

``` python
df = pd.DataFrame({'A':[1,3,2,5,4,7,6], 'B':[1,1,1,1,2,2,2]})
fig, ax = plt.subplots()
df.boxplot('A', 'B', ax=ax)
```

I cannnot change the plot anymore via `ax`, e.g. `ax.set_ylim(0,20)` or `ax.set_ylabel("Something")` has no effect on the figure.

Complete example: http://nbviewer.ipython.org/5868420/bug_pandas-boxplot-set-ylim.ipynb

I tested it on the dev version of pandas (and I know it worked previously, but cannot check which version exactly anymore).
