public function exchange_token($client_id, $client_secret, $redirect_uri, $code = '') {
		$params = array(
			'grant_type' => 'authorization_code',
			'client_id' => $client_id,
			'client_secret' => $client_secret,
			'redirect_uri' => $redirect_uri,
			'code' => $code
		);
		return $this->_request('oauth', $params, self::OAUTH_TOKEN_URL);
	}