Added support for akka cluster configuration and made it optional.
Added support for leader and some initial hooks for UI to work.
Fixed local cluster mode for tests.
Merge branch 'akka-cluster-standalone-mode' of github.com:eklavya/incubator-spark into akka-cluster-standalone-mode

Conflicts:
	core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

    This should fix the broken UI.
Fixed compilation issues post merge.
Ui changes for leader and workers.