Checks for local BigCommerce authentication token, and stores a new one if needs be

    /*
     * Get the auth token from local option
     *
     * @access public
     * @return NULL
     */
    public function get_local_auth_token() {

        // get the token from the options table
        $this->auth_token = get_option( 'bigcommerce_auth_token' );

        // if the local copy is empty, go get one from BigCommerce
        if ( empty( $this->auth_token ) ) {

            // get the token
            $token = $this->get_remote_auth_token();

            // set the local copy for next time
            update_option( 'bigcommerce_auth_token', $token );

            // set the token for this instance
            $this->auth_token = $token;

        }

    }