fix for https://spark-project.atlassian.net/browse/SPARK-1052

Author: Bijay Bisht <bijay.bisht@gmail.com>

Closes #568 from bijaybisht/SPARK-1052 and squashes the following commits:

da70395 [Bijay Bisht] fix for https://spark-project.atlassian.net/browse/SPARK-1052 - comments incorporated
fdb1d94 [Bijay Bisht] fix for https://spark-project.atlassian.net/browse/SPARK-1052

(cherry picked from commit e797c1abd9692f1b7ec290e4c83d31fd106e6b05)
Signed-off-by: Aaron Davidson <aaron@databricks.com>
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