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 ``NotificationFeed`` constructor can throw this, it does a bunch of ``Preconditions.checkArgument`` calls. I'll update the javadoc so that it's more explicit.