Bug 1025872 — Setup websockets for call setup information

The API has been defined by @adamroach in this document: https://wiki.mozilla.org/Loop/Architecture/MVP#WebSockets_Connection_for_Call_Progress

- [x] Implement web socket support;
- [x] Implement the protocol defined in the documentation;
- [x] Support timers and automatically disconnect;
- [x] Have the timers on the same conf key;
- [x] Set the state before returning it, not after;
- [x] Provide a valid authentication mechanism for Hawk;
- [x] Be sure we don't leak listeners on the pubsub;
- [x] Add documentation;
- [x] Fix the travis tests;
- [x] Update the loadtests.