Defined two functions that return the library name and version string.
Always acknowledge confirmable responses.

Instead of letting the receipient acknowledge a confirmable response in
its custom response handler, this is now done automatically.
Support for response code 7.31 to cancel observation relationships.

This new behavior is described in Section 4.6 of draft-ietf-core-observe-12.
The idea is to send a response with code 7.31 and the token to identify
the subscription.
bugfix: #24 Empty ACK improperly includes Token

Included the patch proposed by tbahls to fix this issue. Now, the
ACK message is always truncated to its bare header when the message code
is 0.00.
bugfix: #25 async example creates short packet

Increased packet size by 5.