[SPARK-2359][MLlib] Correlations

Implementation for Pearson and Spearman's correlation.
This feature actually makes it error-prone to add new correlations. For example, I use `corr(x, y, "p")` in my code and in v1.2 someone adds another correlation starting with "p". Then it breaks existing code. I saw R implements this feature but it simply limits the methods to pearson, spearman, and kendall. If they plan to add new correlation with prefix collision, they will face the same problem.