Add a CoAP response node

[JKRhb]

This PR will add a reponse node to the package that is similar to the HTTP response node in the core package.

At the moment, as it was discussed in issue #14, sending an actual response after an incoming request requires a function node which calls msg.res.end() which is rather unintuitive. Having a dedicated reponse node will make flows more easy to understand. It will also enable the package to implement more features from the specifications, for example the handling of different media types and options from requests.

This PR is still WIP. Once it is finished it will resolve #20.

[coveralls]

Pull Request Test Coverage Report for Build 2085815430

• 45 of 62 (72.58%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-9.8%) to 80.08%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
coap/coap-in.js	45	62	72.58%

Totals
Change from base Build 2082357389:	-9.8%
Covered Lines:	140
Relevant Lines:	167

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💛 - Coveralls