[SPARK-5843] [API] Allowing map-side combine to be specified in Java.

Specifically, when calling JavaPairRDD.combineByKey(), there is a new
six-parameter method that exposes the map-side-combine boolean as the
fifth parameter and the serializer as the sixth parameter.

Author: mcheah <mcheah@palantir.com>

Closes #4634 from mccheah/pair-rdd-map-side-combine and squashes the following commits:

5c58319 [mcheah] Fixing compiler errors.
3ce7deb [mcheah] Addressing style and documentation comments.
7455c7a [mcheah] Allowing Java combineByKey to specify Serializer as well.
6ddd729 [mcheah] [SPARK-5843] Allowing map-side combine to be specified in Java.