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.
I am a bit refused to extend a class simply you want to use a method from it. Probably the whole class hierarchy is not that right. If what it does is simply `GSON.fromJson`, better not to extends from the app fabric http handler but do it directly in this class.