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.
Oh ok I didn't get the comment. I moved ``TempFolder.java`` from appfabric to cdap-common-unit-test. ``TempFolder`` is used in a ``AppFabricTestHelper`` class that is not in its test directory, for some reason. So that's why.