Refactory: Message passing layer, Application actor hiearachy 

1. TaskActor no longer need to find out downstream actor and upstream actor address. This is all handled by Express message passing layer.
2. Now each task will be grandchild of AppMaster, instead of reporting to worker.
3. Add a SendLater actor in executor. When the connection link between executors are not established, messages will be stored temporary at SendLater actor.
4. Remote ControlMessage in netty layer
5. Use 8 bytes long to uniquely identify a task. 
6. Rework the worker to make it generic. Now, worker no longer have any assumption on executor. It is just a process.
7. remove class ExpressAddress, now it is replaced with (hostPort, taskId)
8. Fix command shell classpath, by setting extraClassPath in sbt pack
9. Configure the default appMaster and executor jvm option in application.conf
10. rename org.apache.gears.cluster.main to org.apache.gearpump.cluster. As gears and gearpump they have many shared class, and the time is not mature yet to seperate them. 
11. Move streaming related code to streaming package.