Switching to new GCM client API (#95, missing exponential back-off)

Adding new dependency: Google Play Services library

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

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Begin using Proguard from 3.0a7

Google Play Services is taking too much space. The proguard
configuration file is not very well tested yet, but it seems to work.

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Back-off implementation adopted from old gcm.jar (UNTESTED, closes #95)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
No need to keep an instance to Random (it was single use)

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