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())
You have to leave an empty line after this line in order that the list will be rendered as a list (in the online html docstring pages). Or other option is to remove the line and just have the list items, then there does not need to be an empty line.