Table grouping and aggregation

This PR provides a basic and simple implementation of Table grouping and aggregation.  For reference see the full-featured [Pandas implementation](http://pandas.pydata.org/pandas-docs/dev/groupby.html).

There are no docs yet since I thought I would wait for initial comments before going too far in that direction, so looking at the tests is a good idea to see what can be done now.  The docstrings could use a little work as well.