Added initial support for coap options Observe, Conditional Observe and High Level States.

As described in
https://datatracker.ietf.org/doc/draft-li-core-conditional-observe/ (Version 04)
https://datatracker.ietf.org/doc/draft-ietf-core-observe/ (Version 06)
https://datatracker.ietf.org/doc/draft-mietz-core-coap-state-option/ (Version 00)
Merge pull request #58 from master-lincoln/for-merge

Added initial support for coap options Observe, Conditional Observe and High Level States.