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())
use ``com.is_timedelta64_dtype`` here (and ``com.is_datetime64_dtype`` above (rather than the sub-class check which is older).