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.


I would describe parameters using the param syntax.  E.g.:
```
@param mu  Means for each Gaussian distribution in the mixture, where mu(i) is the mean for Gaussian i.
```