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 don't need to use keys
that's the problem , just construct the sub frames and concat glues them together in the same order