Eliminate need for GET_TASKS permission

* Don't check if we are in the foreground - that happens anyway in the
sendExtras() method (https://github.com/phonegap-build/PushPlugin/blob/master/src/android/com/plugin/gcm/PushPlugin.java#L111)
* Only display a message if the message has text to be displayed

Fixes #52 and undoes #3