use sockjs to talk to the browser

If front is going to open a real-time connection to the browser, it should probably be based on a library that falls back to other options when websockets aren't supported.

SockJS seems like the best option (https://github.com/sockjs/sockjs-node).
Last commit was a month ago, but the project is unchanging since it's stable, not because it's abandoned. Meteor (http://meteor.com) used socket.io at first, but found it buggy and switched to sockjs.

For the future - we should tell sockjs to avoid using the websocket transport on mobile clients (see https://speakerdeck.com/3rdeden/realtimeconf-dot-oct-dot-2012)