Added a unit test for PairRDDFunctions.lookup

Lookup didn't have a unit test. Added two tests, one for with a partitioner, and one for without.

Author: Bryn Keller <bryn.keller@intel.com>

Closes #36 from xoltar/lookup and squashes the following commits:

3bc0d44 [Bryn Keller] Added a unit test for PairRDDFunctions.lookup