Update to Akka 2.3.4 and add persistence

* Use ClusterSingletonProxy from frontend
* Remove MultiMaster to reduce complexity
* Make master state persistent
* Improve the Main class and describe how to start
  different parts in separate terminal windows
Merge pull request #8 from typesafehub/wip-update-to-akka-2.3.4-patriknw

Update to Akka 2.3.4 and add persistence