Add textsecure.MessageReceiver

Encapsulate the websocket resources and socket setup process in a
friendly OO class. The MessageReceiver constructor expects an instance
of EventTarget on which to fire message events asynchronously. The
provider of the EventTarget can then add/remove listeners as desired.

Gruntfile.js

@@ -56,6 +56,7 @@ module.exports = function(grunt) {
           'libtextsecure/helpers.js',
           'libtextsecure/stringview.js',
           'libtextsecure/api.js',
+          'libtextsecure/message_receiver.js',
           'libtextsecure/sendmessage.js',
         ],
         dest: 'js/libtextsecure.js',