Update log4js module to expose a shutdown function.

loadAppender will check for a shutdown function exposed by
a loaded appender. If present, it will be cached so that the
shutdown function can execute it.

The intent here is that a Node application would not invoked
process.exit until after the log4js shutdown callback returns.

lib/log4js.js

@@ -56,6 +56,7 @@ var events = require('events')
 , appenders = {}
 , loggers = {}
 , appenderMakers = {}
+, appenderShutdowns = {}
 , defaultConfig =   {
   appenders: [
     { type: "console" }
@@ -299,24 +300,23 @@ function loadAppender(appender) {
     appenderModule = require(appender);
   }
   module.exports.appenders[appender] = appenderModule.appender.bind(appenderModule);
+  if (appenderModule.shutdown) {
+    appenderShutdowns[appender] = appenderModule.shutdown.bind(appenderModule);
+  }
   appenderMakers[appender] = appenderModule.configure.bind(appenderModule);
 }
 
 function shutdown(cb) {
-  var allAppenders = Object.keys(appenders).reduce(
+  var shutdownFunctions = Object.keys(appenderShutdowns).reduce(
     function(accum, category) {
-      return accum.concat(appenders[category]);
+      return accum.concat(appenderShutdowns[category]);
     }, []);
   async.forEach(
-    allAppenders,
-    function(appender, done) {
-      if (appender.shutdown) {
-        appender.shutdown(done);
-      } else {
-        done();
-      }
+    shutdownFunctions,
+    function(shutdownFn, done) {
+      shutdownFn(done);
     },
-    function() { process.nextTick(cb); }
+		cb
   );
 }