SPARK-2978. Transformation with MR shuffle semantics

I didn't add this to the transformations list in the docs because it's kind of obscure, but would be happy to do so if others think it would be helpful.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #2274 from sryza/sandy-spark-2978 and squashes the following commits:

4a5332a [Sandy Ryza] Fix Java test
c04b447 [Sandy Ryza] Fix Python doc and add back deleted code
433ad5b [Sandy Ryza] Add Java test
4c25a54 [Sandy Ryza] Add s at the end and a couple other fixes
9b0ba99 [Sandy Ryza] Fix compilation
36e0571 [Sandy Ryza] Fix import ordering
48c12c2 [Sandy Ryza] Add Java version and additional doc
e5381cd [Sandy Ryza] Fix python style warnings
f147634 [Sandy Ryza] SPARK-2978. Transformation with MR shuffle semantics