API: Return axes from boxplot

Closes: https://github.com/pydata/pandas/issues/4264
Will go in place of https://github.com/pydata/pandas/pull/4472

In https://github.com/pydata/pandas/issues/985 we intentionally decided to have boxplot return a dict with the Lines of the boxplot.

I've added a kwarg `return_dict`. By default, `return_dict` is False. In this case only a `matplotlib.axes` is returned. When `return_dict` is True, we should either

- return the `matplotlib.axes` **and** the dict in a namedtuple
- return just the dict

Both of these are API breaking. Right now I've got both in the named tuple, but I may change that to just return just the dict. If we just return the dict, the only change people will have to make to their code is to add `return_dict` to True in their boxplot calls. Thoughts?

cc @jseabold, @fonnesbeck