Erbium CoAP cleanup for Proposed Standard version

I cleaned up the obsolete draft version since CoAP is a Proposed Standard now (coap-18 will become the RFC). The Erbium (Er) CoAP implementation is now in ./apps/er-coap/ and the RESTful abstraction in ./apps/rest-engine/.

We would also need a clean-up of the old "rest-coap", which is something between draft-ietf-core-coap-02/03. Best would be to eventually have the HTTP implementation for the REST Engine... (sorry no time for that... :( but there have been enough interested parties).