Docs for lurking groupby methods
see [this SO question](http://stackoverflow.com/questions/18104704/counting-unique-values-in-a-pandas-grouped-object)

related: http://stackoverflow.com/questions/21763660/api-ref-missing-dataframegroupby-objects

methods like `nunique` should be documented; i bet there are more like this that you can see in tab completion but that are valid method calls.
