Merge pull request #108 from alig/master

Changes to enable executing by using HDFS as a synchronization point between driver and executors, as well as ensuring executors exit properly.

(cherry picked from commit bab496c1201305f8ee4b0cfdd9b9e8041ea08d38)
Signed-off-by: Reynold Xin <rxin@apache.org>

Conflicts:
	core/src/main/scala/org/apache/spark/SparkContext.scala
Merge branch 'branch-0.8' of https://git-wip-us.apache.org/repos/asf/incubator-spark into branch-0.8