Alternate groupby API that is more functionally consistent with databases or systems like dplyr

[wesm]

pandas's row indexes introduces a level of semantic incompatibility with other systems that occasionally causes problems for users who are using both pandas and some other system.

Functionally, this mainly means returning the group keys as data columns rather than row index. In the case of .apply, it may make sense to discard the group keys altogether.

We may also discuss a means to make specifying more complex aggregations easier in a separate issue

[jreback]

related to this, we might make groupby().filter() a lazy method with returned a filtered groupby (rather than an imemdiate evaluation)

[wesm]

definitely, re: #7