Add google-play-services_lib
Add GCM Client sample app

Taken from https://code.google.com/p/gcm/
Implement basic Helpshift support in GcmClient
Add push handling for notifications from origin helpshift
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  Add push handling for notifications from origin helpshift
  Implement basic Helpshift support in GcmClient
  Add GCM Client sample app
  Add google-play-services_lib