Simple description and code samples (and sample data) for GaussianMixture

Author: Travis Galoppo <tjg2107@columbia.edu>

Closes #4401 from tgaloppo/spark-5013 and squashes the following commits:

c9ff9a5 [Travis Galoppo] Fixed link in mllib-clustering.md Added Gaussian mixture and power iteration as available clustering techniques in mllib-guide
2368690 [Travis Galoppo] Minor fixes
3eb41fa [Travis Galoppo] [SPARK-5013] Added documentation and sample data file for GaussianMixture