Groupbydocs

Addresses #6944, by defining whitelisted methods in the `DataFrameGroupBy` and `SeriesGroupBy` class definitions.  After this change, users of the whitelisted methods now get useful results from `help`, argument name completion, and auto-generated api docs.  These `DataFrameGroupBy` and `SeriesGroupBy` methods are added to the api doc.  Existing unit tests are extended to check for the presence of the whitelisted methods in the class definitions, and a new unit test verifies that old and new paths produce the same results for methods supporting level, axis, skipna arguments.  (Existing methods already explicitly defined in `GroupBy` or derived classes are not overwritten.)

can you move this to ``pandas/utils/decorators.py``? or somewhat similar. doesn't really belong in here as its generic and not related to groupby.