[SPARK-2359][MLlib] Correlations

Implementation for Pearson and Spearman's correlation.

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

Closes #1367 from dorx/correlation and squashes the following commits:

c0dd7dc [Doris Xin] here we go
32d83a3 [Doris Xin] Reviewer comments
4db0da1 [Doris Xin] added private[stat] to Spearman
b716f70 [Doris Xin] minor fixes
6e1b42a [Doris Xin] More comments addressed. Still some open questions
8104f44 [Doris Xin] addressed comments. some open questions still
39387c2 [Doris Xin] added missing header
bd3cf19 [Doris Xin] Merge branch 'master' into correlation
6341884 [Doris Xin] race condition bug squished
bd2bacf [Doris Xin] Race condition bug
b775ff9 [Doris Xin] old wrong impl
534ebf2 [Doris Xin] Merge branch 'master' into correlation
818fa31 [Doris Xin] wip units
9d808ee [Doris Xin] wip units
b843a13 [Doris Xin] revert change in stat counter
28561b6 [Doris Xin] wip
bb2e977 [Doris Xin] minor fix
8e02c63 [Doris Xin] Merge branch 'master' into correlation
2a40aa1 [Doris Xin] initial, untested implementation of Pearson
dfc4854 [Doris Xin] WIP