Merge github.com:apache/spark into cleanup

Conflicts:
	core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala
	core/src/main/scala/org/apache/spark/util/JsonProtocol.scala
Address Patrick's comments
Merge branch 'state-cleanup' of github.com:tdas/spark into cleanup
Make automatic cleanup configurable (not documented)
Merge pull request #3 from andrewor14/cleanup

Patrick's comments