ENH: implement DataFrameGroupBy.boxplot(), close #1507
I noticed this was marked for milestone 0.9.0, so a pull request iso push myself.
I know plenty of goodies are on it`s way for plotting, so this might be a temporary solution (it is not my intention to run in the way of @orbitfold).

Thanks. I think this doesn't do much harm while the plotting API is worked on
