use express 4 router

Refer to http://expressjs.com/4x/api.html#router and http://scotch.io/tutorials/javascript/learn-to-use-the-new-router-in-expressjs-4

- [ ] Generalise handling of DB errors and 404's (to avoid repeating this: https://github.com/jpbochi/duelo/blob/79ece706bb756abb9ba58c038fc9aa22c6730bb0/lib/server/api/games.js#L20-L22)
- [ ] Research how to extract the game lobby/storage service from the "mothership"