Added pruntSplits method to RDD.
Added an extra assert for split size check.
Updated assert == to ===.
Created a PruneDependency to properly assign dependency for
SplitsPruningRDD.
Marked prev RDD as transient in SplitsPruningRDD.
Removed pruneSplits method in RDD and renamed SplitsPruningRDD to
PartitionPruningRDD.
Updated PruneDependency to change "split" to "partition".
Merge pull request #409 from rxin/splitpruningrdd

Added pruntSplits method to RDD.