title: send push notification in android from device to another
body: <p>I am working on a cab booking  app . I am developping two applications the first one is B2C side (for the client who 's going to book a taxi) and B2U side (for the driver of the taxi).I need when a client make a reservation and the taxi driver accept it ,can both communicate with each other by sending notifications for example :the driver can send to the client  "i am about to arrive " etc and vice versa . I need a simple   way  to do this  apart of GCM because it's not totally free .I need an easy way without the need of developping a web service and make my own server .  </p>