db: moved DataSourceDb to JsonObjectDb

JsonObjectDb is a generic db for simple json objects.
client: added simple ClientManager
esper: convert EventBean to JsonNode before sending to listeners
event: added basic event manager
notifications: added basic framework
notifications: updated framework with multiple packages
notifications: merged ClientEplMapper and NotificationManager
client: removed package

Notification manager and client handle that now
test: added client tests
test: slight more coverage in clients
notifications: forward eventId to NotificationSender
notifications: added GcmSender
notifications: added SenderResult handling to EsperUpdateListener
rest: added restlet package
rest: added notification registration restlet
rest: added notification unregistration restlet
Fixed some compiler errors
rest: added NotificationsRestApi
notifications: added googleApi.key.template
test: added tests for notifications.sender