[SPARK-7535] [.1] [MLLIB] minor changes to the pipeline API

1. removed `Params.validateParams(extra)`
2. added `Evaluate.evaluate(dataset, paramPairs*)`
3. updated `RegressionEvaluator` doc

jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #6392 from mengxr/SPARK-7535.1 and squashes the following commits:

5ff5af8 [Xiangrui Meng] add unit test for CV.validateParams
f1f8369 [Xiangrui Meng] update CV.validateParams() to test estimatorParamMaps
607445d [Xiangrui Meng] merge master
8716f5f [Xiangrui Meng] specify default metric name in RegressionEvaluator
e4e5631 [Xiangrui Meng] update RegressionEvaluator doc
801e864 [Xiangrui Meng] Merge remote-tracking branch 'apache/master' into SPARK-7535.1
fcbd3e2 [Xiangrui Meng] Merge branch 'master' into SPARK-7535.1
2192316 [Xiangrui Meng] remove validateParams(extra); add evaluate(dataset, extra*)