Timeout zombie workers

Conflicts:
	core/src/main/scala/spark/deploy/master/Master.scala
milliseconds -> seconds in timeOutDeadWorkers logging
Moved handling of timed out workers within the Master actor

Conflicts:
	core/src/main/scala/spark/deploy/DeployMessage.scala
	core/src/main/scala/spark/deploy/master/Master.scala
Merge pull request #12 from markhamstra/0.8.0-candidate-csd-6

Cherry-pick bug-fixes from AMPLab repo