public function refresh_token() {
		if (!isset($this->auth_details['refresh_token']))
			trigger_error('Error refreshing token. There is no refresh token set on this object.', E_USER_ERROR);
		$params = array(
			'grant_type' => 'refresh_token',
			'refresh_token' => $this->auth_details['refresh_token']
		);
		return $this->_request('oauth', $params, self::OAUTH_TOKEN_URL);
	}