SPARK-4156 [MLLIB] EM algorithm for GMMs

Implementation of Expectation-Maximization for Gaussian Mixture Models.

This is my maiden contribution to Apache Spark, so I apologize now if I have done anything incorrectly; having said that, this work is my own, and I offer it to the project under the project's open source license.


minor: GMM's name contains `Model`, which is a little confusing: `GaussianMixtureModelEM` produces `GaussianMixtureModel`. I don't have good suggestions. Maybe we could rename `GaussianMixtureModelEM` to `GaussianMixtureEM`.