[SPARK-2359][MLlib] Correlations

Implementation for Pearson and Spearman's correlation.
FWIW I tend to strongly agree that the algorithm should be specified by exactly one complete string. R isn't necessarily something to emulate. Even using Strings feels a bit old-school. I understand possible cross-platform concerns but what about passing in implementations of a Correlation interface, which are available as singleton instances for convenience from somewhere? Food for thought.