Added Observe option handling for server.

* Updated coap-packet to 0.1.8
* Added a ObserveWriteStream for streaming updates
* Extracted some helpers out of OutgoingMessage to avoid code
  duplication in ObserveWriteStream
* Changed signature of send callback passed to OutgoingMessage
* Added converters for the Observe option (not fully tested)
Added Observe support for request.
Fixed Copper compatibility.
Added Observe client and server example.
Added a finish event after closing the write stream.
Some docs.
More readme fixes.
README tweaks.
Merge pull request #5 from mcollina/observe

Observe support