Aggregated `offer`,`answer`,`denied` message types into `roap-protocol`

Helps keeping the webrtc signaling consistent with other protocols and
makes clear the relationship between the different messages

* `offer`,`answer` and `denied` types are now called `roap-protocol` (roap is defined in this [Draft](http://tools.ietf.org/html/draft-jennings-rtcweb-signaling-01).
* updated tests and mocks
* updated bootstrap servers

To be fully roap-compliant we would have to exchange the denied type with the appropriate error type ([Draft](http://tools.ietf.org/html/draft-jennings-rtcweb-signaling-01, Sec 5.6.)