Fixup delivery receipts

Uses app-level timestamps for outgoing messages.
Adds timestamp property to the outgoing jsonData.
Triggers a runtime event to notify frontend on delivery receipts.
Renders delivered messages with a 'delivered' class.

js/api.js

@@ -215,6 +215,7 @@ window.textsecure.api = function () {
         var jsonData = { messages: messageArray };
         if (messageArray[0].relay !== undefined)
             jsonData.relay = messageArray[0].relay;
+        jsonData.timestamp = messageArray[0].timestamp;
 
         return doAjax({
             call                : 'messages',