[SPARK-7404] [ML] Add RegressionEvaluator to spark.ml

Author: Ram Sriharsha <rsriharsha@hw11853.local>

Closes #6344 from harsha2010/SPARK-7404 and squashes the following commits:

16b9d77 [Ram Sriharsha] consistent naming
7f100b6 [Ram Sriharsha] cleanup
c46044d [Ram Sriharsha] Merge with Master + Code Review Fixes
188fa0a [Ram Sriharsha] Merge branch 'master' into SPARK-7404
f5b6a4c [Ram Sriharsha] cleanup doc
97beca5 [Ram Sriharsha] update test to use R packages
32dd310 [Ram Sriharsha] fix indentation
f93b812 [Ram Sriharsha] fix test
1b6ebb3 [Ram Sriharsha] [SPARK-7404][ml] Add RegressionEvaluator to spark.ml