Added ability to dismiss notifications remotely

Just send a push with the GCM variable cancelNotification set (can be set to anything) and the notification will be dismissed. Default functionality uses the standard NOTIFICATION_ID=237.

The code addition also works with the the code from TheNewMR (https://github.com/thenewmr/PushPlugin/commit/728e975442e45668b7d809aec028396a9b023dfa) which allows for stacked notifications depending on different notification ID's. To dismiss a Notification ID just make sure to set the GCM variable cancelNotification and to set notId to the notification ID you want to dismiss.