Add support for coaps:// scheme / secure CoAP

[kartben]

[mkovatsc]

The DTLS part is unfortunately non-scriptable. Internatlly, it is quite convoluted with ICE and the WebRTC code, so it is not straight-forward to make it scriptable. Thus, one needs to implement DTLS in JavaScript. On the one hand, this is quite a pain in the a**, on the other hand, it could work out fine when using the browser crypto APIs.

Happy for any volunteer :) If you---the volunteer---are a student motivated and capable enough, I could also set this up as a master's thesis...