title: Google Cloud Messaging send notification to all devices
body: <p>I'm new in GCM. I would like to send an message to all devices that have the app installed. I read about registration_id: after the first connection to GCM, google send this unique string to device. I'm a beginner in server world but if I'm not mistaken, in server side, for sending a notification to devices I have to send array of registration_id and the message to google.</p>
  
  <p>Google knows how has the registration id?
  Is there a way to send messages to all devices without pass the registrarions id?
  Thank you.</p>