[SPARK-6113] [ML] Tree ensembles for Pipelines API

This is a continuation of [https://github.com/apache/spark/pull/5530] (which was for Decision Trees), but for ensembles: Random Forests and Gradient-Boosted Trees.  Please refer to the JIRA [https://issues.apache.org/jira/browse/SPARK-6113], the design doc linked from the JIRA, and the previous PR linked above for design discussions.

This PR follows the example set by the previous PR for Decision Trees.  It includes a few cleanups to Decision Trees.

Note: There is one issue which will be addressed in a separate PR: Ensembles' component Models have no parent or fittingParamMap.  I plan to submit a separate PR which makes those values in Model be Options.  It does not matter much which PR gets merged first.

CC: mengxr manishamde codedeft chouqin

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

Closes #5626 from jkbradley/dt-api-ensembles and squashes the following commits:

729167a [Joseph K. Bradley] small cleanups based on code review
bbae2a2 [Joseph K. Bradley] Updated per all comments in code review
855aa9a [Joseph K. Bradley] scala style fix
ea3d901 [Joseph K. Bradley] Added GBT to spark.ml, with tests and examples
c0f30c1 [Joseph K. Bradley] Added random forests and test suites to spark.ml.  Not tested yet.  Need to add example as well
d045ebd [Joseph K. Bradley] some more updates, but far from done
ee1a10b [Joseph K. Bradley] Added files from old PR and did some initial updates.

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