Proof of concept `server` command

Uses [reactphp](http://reactphp.org) as a concurrent socket server and creates
a server/worker model for handler HTTP requests.

The worker process isn't reused so global state doesn't matter and all of the
`include_once` and `require_once` won't cause any issues.

To see your WP site's 404 page HTML run:

    `wp server`

And then go to [http://localhost:4000/](http://localhost:4000)