Consolidated both mapPartitions related RDDs into a single MapPartitionsRDD.
Also changed the semantics of the index parameter in mapPartitionsWithIndex from the partition index of the output partition to the partition index in the current RDD.
Update tuning.md

Clarify when serializer is used based on recent user@ mailing list discussion.
Merge pull request #201 from rxin/mappartitions

Use the proper partition index in mapPartitionsWIthIndex

mapPartitionsWithIndex uses TaskContext.partitionId as the partition index. TaskContext.partitionId used to be identical to the partition index in a RDD. However, pull request #186 introduced a scenario (with partition pruning) that the two can be different. This pull request uses the right partition index in all mapPartitionsWithIndex related calls.

Also removed the extra MapPartitionsWIthContextRDD and put all the mapPartitions related functionality in MapPartitionsRDD.
Merge pull request #206 from ash211/patch-2

Update tuning.md

Clarify when serializer is used based on recent user@ mailing list discussion.