Support add_relation in GUI Go env

[Gary Poster]

    /**
     * Add a relation between two services.
     *
     * @method add_relation
     * @param {Object} endpointA An array of [service, interface] representing
         the first endpoint to connect.
     * @param {Object} endpointB An array of [service, interface] representing
         the second endpoint to connect.
     * @param {Function} callback A callable that must be called once the
         operation is performed.
     * @return {undefined} Sends a message to the server only.
     */
    add_relation: function(endpointA, endpointB, callback) {
      this._send_rpc({
        'op': 'add_relation',
        'endpoint_a': endpointToName(endpointA),
        'endpoint_b': endpointToName(endpointB)}, callback, true);
    },