Add libtextsecure support for syncing read messages

Plumbing for sending and receiving a new sync protobuf for marking
messages read on/from my other devices.

// FREEBIE

js/background.js

@@ -103,6 +103,7 @@
         messageReceiver.addEventListener('contact', onContactReceived);
         messageReceiver.addEventListener('group', onGroupReceived);
         messageReceiver.addEventListener('sent', onSentMessage);
+        messageReceiver.addEventListener('read', onReadReceipt);
         messageReceiver.addEventListener('error', onError);
 
         window.textsecure.messaging = new textsecure.MessageSender(SERVER_URL, USERNAME, PASSWORD, ATTACHMENT_SERVER_URL);
@@ -228,6 +229,12 @@
         throw e;
     }
 
+    function onReadReceipt(ev) {
+        var timestamp = ev.timestamp.toNumber();
+        var sender    = ev.sender;
+        console.log('read receipt ', sender, timestamp);
+    }
+
     // lazy hack
     window.receipts = new Backbone.Collection();