add RTCPeerConnection as Ros transport library

[aisch]

closes #223

not sure if this is best way to add this support to roslibjs but with this patch i was able to connect a RTCDataChannel to a rosbridge like e.g.

ros.transportLibrary = peer_connection;
ros.transportOptions = {
  reliable: false,
  ordered: false,
};
ros.connect('rosbridge');

[aisch]

@rctoris thanks!

[Interpause]

https://github.com/RobotWebTools/rosbridge_suite does not seem to have any ICE server implementation for WebRTC, how is this feature intended to be used? I am planning to use WebRTC's DataChannel as well, and would not mind attempting to follow the rosbridge protocol, but I am unclear what the current implementation status is.