Readme: Updated the text to introduce Hobnob and it's goal.

* Added a small TODO.
hobnob: Use the constant instead of hardcoding the port number
coap-client: Add function to 'ping' a server

* A coap ping is an empty confirmable message. A server on receiving such
  a message should respond with a Reset message.
package: Export coap:ping-server function
coap: Add function to get a packet-header structure from a sequence

* Cleanup existing helper function names