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())
I added a top sentence at the top of the paragraph (hard to spot in the commit, i know), did you see it and think it's not enough ?
(because basically, all the paragraph now refers to _output_ dtypes, since it's configurable)
Thanks !