Fixes for AGPUSH516 and AGPUSH517

These changes give the desired behavior.  The only outstanding issue I have is to have the app call a method on the PushPlugin to "reinitialize" itself.  For example, re-establish the webView and the javascriptCallback.  According to the GCM docs, we should not be registering very often.  As far as I can tell, we only need to register once.  I am planning on registering upon login since once my user logs in they stay logged in forever.   Likewise, according to the docs, we should only unregister upon app uninstall.   I am planning on unregistering when the user logs out since they most likely will stay logged in for a long period of time.  

Do you know if the Push Server will check to see if the device is already registered and if so will it avoid reregistering?

Your thoughts on all this?