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

CoAP draft v13 promoted the Token Option to a field following the CoAP Header. However the function `coap_get_header_token()` has not been updated to take into account this change. As a consequence the function behaves incorrectly (it always returns 0 and does not retrieve the token value).
This commit fixes the bug and removes every reference to the old Token Option.