Great refactor of 2014. Closes #3.

Broke apart `index.js` into several components. `server.js` is the new entry point; and is responsible for setting up:

* A web server that...
  * ...consumes GitHub webhooks.
  * ...reports on the status of the server (for the load balancer).
* `Queue`, which watches for changes to the firebase queue, synchronizing ownership over queue items, and dispatching them to be processed.
* `TestRunner`s are spawned to process commits. Most of the logic in `index.js` migrated here. Albeit flattened.