[SPARK-1784] Add a new partitioner to allow specifying # of keys per partition

This change adds a new partitioner which allows users
to specify # of keys per partition.

Author: Syed Hashmi <shashmi@cloudera.com>

Closes #721 from syedhashmi/master and squashes the following commits:

4ca94cc [Syed Hashmi] [SPARK-1784] Add a new partitioner