Zina - checkpoint 7

GCM implementation added and messaging possible

In this exercise, we are going to implement sending and receiving messages by using HTTP calls as well as downstream messaging and receiving messages via a BroadcastReceiver. As we already have our database set up and our friends are stored in it, we can send offline messages – we can type them whenever we want and they will be sent once the device goes online. For that, we can register another BroadcastReceiver, but this time it will be listening to a different type of an event – connection status. Once BroadcastReceiver receives online connection event, it will start a service that will retrieve all unsent messages and send them over.