[SPARK-8054] [MLLIB] Added several Java-friendly APIs + unit tests

Java-friendly APIs added:
* GaussianMixture.run()
* GaussianMixtureModel.predict()
* DistributedLDAModel.javaTopicDistributions()
* StreamingKMeans: trainOn, predictOn, predictOnValues
* Statistics.corr
* params
  * added doc to w() since Java docs do not inherit doc
  * removed non-Java-friendly w() from StringArrayParam and DoubleArrayParam
  * made DoubleArrayParam Java-friendly w() actually Java-friendly

I generated the doc and verified all changes.

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #6562 from jkbradley/java-api-1.4 and squashes the following commits:

c16821b [Joseph K. Bradley] Small fixes based on code review.
d955581 [Joseph K. Bradley] unit test fixes
29b6b0d [Joseph K. Bradley] small fixes
fe6dcfe [Joseph K. Bradley] Added several Java-friendly APIs + unit tests: NaiveBayes, GaussianMixture, LDA, StreamingKMeans, Statistics.corr, params