Notifications System core

Following https://issues.cask.co/browse/CDAP-849 jira and design doc.
Build is running at https://builds.cask.co/browse/CDAP-DUT320-3.
The ``Store`` is maintained by app fabric, so by using the handler I feel like we decouple things more than if we had a dependency on app fabric in the notification module. I feel like a lot of our code is already like that. Isn't decoupling a good reason enough?