Simplify SubtractedRDD in preparation from subtractByKey.
Finished subtractByKeys.
Allow subtractByKey's other argument to have a different value type.
Fix tabs that snuck in.
Merge pull request #525 from stephenh/subtractByKey

Add PairRDDFunctions.subtractByKey.
Merge branch 'master' of https://github.com/mesos/spark into foldByKey
refactor foldByKey to use combineByKey