[SPARK-4728][MLLib] Add exponential, gamma, and log normal sampling to MLlib da...

...ta generators

This patch adds:

* Exponential, gamma, and log normal generators that wrap Apache Commons math3 to the private API
* Functions for generating exponential, gamma, and log normal RDDs and vector RDDs
* Tests for the above

Author: RJ Nowling <rnowling@gmail.com>

Closes #3680 from rnowling/spark4728 and squashes the following commits:

455f50a [RJ Nowling] Add tests for exponential, gamma, and log normal samplers to JavaRandomRDDsSuite
3e1134a [RJ Nowling] Fix val/var, unncessary creation of Distribution objects when setting seeds, and import line longer than line wrap limits
58f5b97 [RJ Nowling] Fix bounds in tests so they scale with variance, not stdev
84fd98d [RJ Nowling] Add more values for testing distributions.
9f96232 [RJ Nowling] [SPARK-4728] Add exponential, gamma, and log normal sampling to MLlib data generators