ENH: Add unique and nunique to GroupBy whitelist
Series.unique and Series.nunique are useful methods for querying the
unique elements (or number of unique elements) in each group when used
in conjunction with DataFrame.GroupBy.  This commit addresses
closes pydata/pandas#6146.

pls add in `tests/test_groupby.py` as well (for the whitelist tests)
 Ok, I added the tests as well.  Let me know if everything looks ok.
 thanks for this!
