Merge branch 'mos-bt'

This merge keeps only the broadcast work in mos-bt because the structure
of shuffle has changed with the new RDD design. We still need some kind
of parallel shuffle but that will be added later.

Conflicts:
	core/src/main/scala/spark/BitTorrentBroadcast.scala
	core/src/main/scala/spark/ChainedBroadcast.scala
	core/src/main/scala/spark/RDD.scala
	core/src/main/scala/spark/SparkContext.scala
	core/src/main/scala/spark/Utils.scala
	core/src/main/scala/spark/shuffle/BasicLocalFileShuffle.scala
	core/src/main/scala/spark/shuffle/DfsShuffle.scala