Mock a the minimal subset of WebRTC for testing.

The intent of this PR is to introduce a basic mocking of the WebRTC API.
That's done in [FirefoxZombieDialect_spec.js](https://github.com/inetCatapult/zombese/blob/feat-webrtc-mock/test/webrtc/dialects/FirefoxZombeseDialect_spec.js). I am checking to make sure the Firefox dialect was used here.