Author: Ram Sriharsha <rsriharsha@hw11853.local>

Closes #6365 from harsha2010/SPARK-7833 and squashes the following commits:

923f288 [Ram Sriharsha] cleanup
7623b7d [Ram Sriharsha] python style fix
9743f83 [Ram Sriharsha] [SPARK-7833][ml] Add python wrapper for RegressionEvaluator