Configable WebRtcTransport class + transport implementation

[iislucas]

WebRTCTransportProvider currently hard-wires STUN servers. People wanting to use WebRtc for transport (e.g. uProxy) would like to configure both STUN servers and employ TURN servers too (#49). These things are specific to WebRTC, so maybe don't want to be in Transport API. But we do want the option to use/specify them...

One way to do this is to make a WebRtc class that uses peer-connection and does all the work, and is configurable. Then the WebRTCTransportProvider is a lightweight wrapper around this class with some reasonable set defaults.

[Paul-E]

Current list of hard coded servers: https://github.com/UWNetworksLab/freedom/blob/v0.3.1/providers/transport/webrtc/transport.webrtc.js#L25