Fix for coap_get_header_token() always returning 0 and not retrieving the token when using CoAP draft v13.
Every reference to the old Token Option has been removed (in CoAP draft v13 the Token Option has been promoted to a field following the CoAP Header).
Merge pull request #298 from alessandrelli/er-coap-13-token-fix

Fix for coap_get_header_token() always returning 0 when using CoAP draft v13