Switch from instanceof to method_exists.

This change lets custom loggers implement scopes without inheriting from
BaseLog.

lib/Cake/Log/CakeLog.php

@@ -428,7 +428,7 @@ public static function write($type, $message, $scope = array()) {
 		foreach (self::$_Collection->enabled() as $streamName) {
 			$logger = self::$_Collection->{$streamName};
 			$types = $scopes = $config = array();
-			if ($logger instanceof BaseLog) {
+			if (method_exists($logger, 'config')) {
 				$config = $logger->config();
 			}
 			if (isset($config['types'])) {