Add subtractByKey to the JavaPairRDD wrapper

Author: Punya Biswal <pbiswal@palantir.com>

Closes #600 from punya/subtractByKey-java and squashes the following commits:

e961913 [Punya Biswal] Hide implicit ClassTags from Java API
c5d317b [Punya Biswal] Add subtractByKey to the JavaPairRDD wrapper