GCM support

[richsage]

Thanks to @mrzepinski for his submission which was used as the basis for this addition :-)

Adds GCM support but keeps BC with C2DM.

NB. This also includes an adjustment to the configuration format for Android. The old configuration will still work for the time being, but if you are using C2DM, please move to the new format:

android:
    c2dm:
        username: foo@example.com
        password: bar
        source: 123
    gcm:
        api_key: ABC123