ENH: more flexible describe() + tests

this patch adds a return_type keyword argument to describe() to make it more
flexible to use on mixed-type dataframe. User can now select among returning
numeric, categorical, or both, as well as 'auto' (previous behaviour, default),
and 'same', which keep columns identical (useful, e.g. with groupby())
Yes that part. I thought as a whole, the docstring made that clear. Ok, i'll update it to make it more explicit. Thanks