Fix gcm validator

Pull request for gcm.

http://developer.android.com/google/gcm/gcm.html

* FIX: PayloadValidator

> 'data' field is a limit on the total size of the message (4kb).

However it validates total size of payload now.

* ADD: RegistrationIds validator

> It must contain at least 1 and at most 1000 registration IDs.