[SPARK-2359][MLlib] Correlations

Implementation for Pearson and Spearman's correlation.
So the fact R has been supporting this feature for many years  (and still haven't deprecated it) is good precedence that among the well known correlations, there aren't really any collisions. Since we require that only well known and widely adopted algorithms be added to mllib, the likelihood of collision is even smaller. (If someone's adding something in their own version of spark, presumably they have already looked closely at the docs and been warned).
Because we decided to go with strings for method specification instead of enums for cross language uniformity, the method is only checked (and possibly invalidated) at run time. Therefore, we do want to provide mechanisms for fault tolerance and minimization in method specification. To address a related comment, "spearman" and "spearmans" are both really common in referring to Spearman's correlation (imagine the heartbreak of having your program fail because of an extra "s" after hours of data manipulation computation!)