Added unpersist method to Broadcast.
Fix for Broadcast unpersist patch.

Updated comment in MemoryStore.dropFromMemory
Keep TorrentBroadcast piece blocks until unpersist is called
Merge branch 'bc-unpersist-merge' of github.com:ignatich/incubator-spark into cleanup

Conflicts:
	core/src/main/scala/org/apache/spark/broadcast/BroadcastFactory.scala
	core/src/main/scala/org/apache/spark/broadcast/TorrentBroadcast.scala
	core/src/main/scala/org/apache/spark/storage/MemoryStore.scala