API: decide what to return from plotting functions
most plotting functions return an `Artist` instance whereas a few return something else. for example, `boxplot` returns a `dict` and `scatter` returns a `tuple`. they should at least return a `matplotlib.artist.Artist` subclass (usually either a `Figure` or `Axes`) or an array of `Artist`s.

this has been brought up before but it would be nice to make a decision here.

[mailing list thread](https://groups.google.com/d/topic/pystatsmodels/biNlCvJPNNY/discussion) and #3474.
