Add RDD.subtract.

Instead of reusing the cogroup primitive, this adds a SubtractedRDD
that knows it only needs to keep rdd1's values (per split) in memory.
Merge branch 'master' into subtract
Update for split->partition rename.
Add subtract to JavaRDD, JavaDoubleRDD, and JavaPairRDD.
Merge pull request #473 from stephenh/subtract

Add RDD.subtract.