notifications: renamed SimpleNotificationFilter to NotificationFilter
notifications: moved all db handling to NotificationManager
notifications: added concept of rest adapters
notifications: RestAdapter uses generic client type
notifications: added concept of NotificationDefinition

A definition links a client type to a sender type, rest adapter
and rest name to be used for the api
notifications: moved DefinitionVisitor content to NotificationManager
notifications: NotificationRouter builds dynamic rest api

... based on registered NotificationDefinitions
notifications: removed debug output
notifications: added framwork for rest notifications
notifications: added logic for basic RestSender
notifications: sender can throw NotificationException

And removed multithreading from GcmSender. Mutlithreading
should not happen on sender level, rather on manager
level.
notifications: fixed bug where notifications were sent for all sources
notifications: handle NPE on missing google api key