Since CrossValidator is a meta algorithm, we copy the implementation in Python. jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #5926 from mengxr/SPARK-6940 and squashes the following commits:

6af181f [Xiangrui Meng] add TODOs
8285134 [Xiangrui Meng] update doc
060f7c3 [Xiangrui Meng] update doctest
acac727 [Xiangrui Meng] add keyword args
cdddecd [Xiangrui Meng] add CrossValidator in Python