[SPARK-2786][mllib] Python correlations

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1713 from dorx/pythonCorrelation and squashes the following commits:

5f1e60c [Doris Xin] reviewer comments.
46ff6eb [Doris Xin] reviewer comments.
ad44085 [Doris Xin] style fix
e69d446 [Doris Xin] fixed missed conflicts.
eb5bf56 [Doris Xin] merge master
cc9f725 [Doris Xin] units passed.
9141a63 [Doris Xin] WIP2
d199f1f [Doris Xin] Moved correlation names into a public object
cd163d6 [Doris Xin] WIP
Merge remote-tracking branch 'upstream/master' into decisiontree-python-new

Conflicts:
	mllib/src/main/scala/org/apache/spark/mllib/api/python/PythonMLLibAPI.scala
(not a real conflict; merged by concatenation)