Add real dumping of persistence-schema to examples/

[LukaszKontowski]

As for now, compiling the example application: examples/akka-cluster-app with dump-persistence-schema-plugin enabled does not produce any dump. We should add a working example for dumping persistence schema.

Still to be decided, which option to choose:
a) Add persistence schema to examples/akka-cluster-app (modify app's logic)
b) Leave examples/akka-cluster-app unchanged and add another example app under examples - just for this need