BUG: Use arrays for week/month group calculations.
BUG: Use arrays for week/month group calculations.

In pandas 0.18, the behavior of ``nth()`` changed so that Grouper no
longer can be easily used to recover group labels.

Instead of using the built-in grouper behavior, we use a groupby on two
arrays we build ourselves.  This recovers the original behavior, and is
about 2x faster as a bonus.

bartosh/zipline,enigmampc/catalyst,florentchandelier/zipline,humdings/zipline,alphaBenj/zipline,quantopian/zipline,humdings/zipline,Scapogo/zipline,alphaBenj/zipline,quantopian/zipline,florentchandelier/zipline,bartosh/zipline,Scapogo/zipline,enigmampc/catalyst