notifier: Use Runtime.addShutdownHook to catch shutdown signals

Use Runtime.addShutdownHook to catch process shutdown
(SIGTERM, SIGINT, SIGHUP) and not sun's proprietary
sun.misc package. Runtime.addShutdownHook is JVM
agnostic and has the advantage of executing the same
if the service was killed or the java process it self.

Change-Id: I71b3a2a54862b9c3ca1fb6d2c32f127b833953de
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=749513
Signed-off-by: Mooli Tayer <mtayer@redhat.com>