MessageEvent: removed message.type default assignment from constructor
 - reason: since getDefaultdestination() always uses the first message.type assignment,
   a call to setType() doesn't override its value (so event is sent on the wrong channel).
Push Notification Plugin - beta2 release
 - plugin now uses MessageEvent event template
 - each provider listens to its own channel
 - new feature: override global provider params with event ones, so that we can send custom one-time notification easily.
 - some code cleanup