improve imports
add code to restart tasks
add task recovery code
update log
fix bug
fix serialization bugs
fix clocking bug
fix clocking
fix bug in tracking the clock
add recovery code
Merge remote-tracking branch 'upstream/master'

Conflicts:
	streaming/src/main/scala/org/apache/gearpump/streaming/AppMaster.scala
fix merge conflict
Merge pull request #79 from clockfly/master

at least once
Merge branch 'master' into scheduler

Conflicts:
	streaming/src/main/scala/org/apache/gearpump/streaming/AppMaster.scala
	streaming/src/main/scala/org/apache/gearpump/streaming/ClusterMessage.scala
	streaming/src/main/scala/org/apache/gearpump/streaming/Executor.scala