Aggregate '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 should be merged into the single type 'roap-protocol' (ROAP is defined in this [Draft](http://tools.ietf.org/html/draft-jennings-rtcweb-signaling-01).
* update tests and mocks
* update 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.)