v2.2.0: GCM using native REST consume
We have replaced the Java/XMPP-based implementation of the GCM connector with a native Mendix implementation that uses the GCM HTTP/REST API. This should resolve the issue w.r.t. failing push notifications on Android due to a CONNECTION_DRAINING error (issue #45). Other responses are now also properly handled (such as 'device unregistered' errors, or responses with a new canonical registration id).