Don't ignore figsize in df.boxplot
Work in progress from PyCon sprints

Fix for https://github.com/pandas-dev/pandas/issues/11959

It really doesn't feel like a proper fix, but I'm not sure what a proper fix would be (without changing matplotlib).  The matplotlib `gca()` ends up calling `figure()`, but doesn't let us pass in a `figsize` to `gca()`.

Any advice? FYI this works, ie it fixes the behavior for the code snippet in the bug report. It would obviously break in multithreaded scenarios, but I'm not sure if that's a consideration or not.