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.


It is more common to write:
```
val p = weights.zip(dists).map { case (weight, dist) => eps + weight * dist.pdf(x) }
```