Add 'rcstream' module for broadcasting recent changes over WebSockets

This change introduces an 'rcstream' module, which provisions a small and
self-contained software stack for broadcasting recent changes using WebSockets.
It is designed to work with the RedisPubSubFeedEngine class in MediaWiki.

The setup consists of the following components:

* A single-file Python server, powered by gevent and socket.io.
  Each instance of such server can serve multiple clients concurrently.

* An upstart task (rcstream/init) that spawns multiple instances of this server.

* Nginx as load-balancer / reverse proxy. Nginx 1.4+ (and therefore, Trusty) is
  required, as earlier versions are not capable of proxying WebSocket
  connections.

Why WebSockets? Admittedly, it is not the world's prettiest protocol. But it is
very widely supported, and it is what users want.

Please see the included README file and the in-line documentation for details
about the setup.

RFCs fulfilled / obsoleted by this change:

* <https://www.mediawiki.org/wiki/Requests_for_comment/Publishing_the_RecentChanges_feed>

Bug: 14045
Bug: 17450
Change-Id: Id819246a947dfe1e32d9fd9c13570858cdaaa39d