[SPARK-7387] [ML] [DOC] CrossValidator example code in Python

Author: Ram Sriharsha <rsriharsha@hw11853.local>

Closes #6358 from harsha2010/SPARK-7387 and squashes the following commits:

63efda2 [Ram Sriharsha] more examples for classifier to distinguish mapreduce from spark properly
aeb6bb6 [Ram Sriharsha] Python Style Fix
54a500c [Ram Sriharsha] Merge branch 'master' into SPARK-7387
615e91c [Ram Sriharsha] cleanup
204c4e3 [Ram Sriharsha] Merge branch 'master' into SPARK-7387
7246d35 [Ram Sriharsha] [SPARK-7387][ml][doc] CrossValidator example code in Python