Moving Google Play stuff to dedicated flavor

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Depend on Google Play Services only in googleplay flavor

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Creating modular push client service (DOES NOT BUILD)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Google Cloud Messaging classes gone in their own flavor

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Abstract billing interface (UNTESTED)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>