Removed removeChannel from DataStore interface.
Refactoring to make more readable.
Fixing hardcoded version number in NotificationHandlerTest.
Moved DefaultChannel to commons.

- Refactored DefaultSimplePushServerTest to accomodate for usage of
  different datastore implementations.
Adding DefaultSimplePushServer tests that used jpa datastore.
Adding RedisSimplePushServerTest.
Adding CouchDBSimplePushServerTest.