Added `ml.feature.Bucketizer` to PySpark.

cc mengxr

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #6124 from brkyvz/ml-bucket and squashes the following commits:

05285be [Burak Yavuz] added sphinx doc
6abb6ed [Burak Yavuz] added support for Bucketizer