added getBufferedLogger function.

This function should be useful when you need to log during async
parallel operations, without having a mess in logs.
For example when you walk asynchronously a directory and you want logs
to be grouped by file.

It returns the same getLogger() object but messages are stored
internally and sent to appenders only when you call the flush() method
on it.

lib/log4js.js

@@ -70,6 +70,29 @@ function hasLogger(logger) {
 }
 
 
+function getBufferedLogger(categoryName) {
+    var base_logger = getLogger(categoryName);
+    var logger = {};
+    logger.temp = [];
+    logger.base_logger = base_logger;
+    logger.flush = function () {
+        for (var i = 0; i < logger.temp.length; i++) {
+            var log = logger.temp[i];
+            logger.base_logger[log.level](log.message);
+            delete logger.temp[i];
+        }
+    };
+    logger.trace = function (message) { logger.temp.push({level: 'trace', message: message}); }
+    logger.debug = function (message) { logger.temp.push({level: 'debug', message: message}); }
+    logger.info = function (message) { logger.temp.push({level: 'info', message: message}); }
+    logger.warn = function (message) { logger.temp.push({level: 'warn', message: message}); }
+    logger.error = function (message) { logger.temp.push({level: 'error', message: message}); }
+    logger.fatal = function (message) { logger.temp.push({level: 'fatal', message: message}); }
+
+    return logger;
+}
+
+
 /**
  * Get a logger instance. Instance is cached on categoryName level.
  * @param  {String} categoryName name of category to log to.