Add optional timezoneOffset config for appenders

Example:
    log4js.configure({
        appenders: [{type: 'console', timezoneOffset: -540}],
        replaceConsole: true
    });

The expected value is the equivalent of (new Date).getTimezoneOffset()
In this example, -540 is the value for JST.
This allows machines members of world-wide-spread cluster to all report
log time-stamps using the same timezone (or adapt the timezone to a
local different from the system)

lib/appenders/console.js

@@ -2,10 +2,10 @@
 var layouts = require('../layouts')
 , consoleLog = console.log.bind(console);
 
-function consoleAppender (layout) {
+function consoleAppender (layout, timezoneOffset) {
   layout = layout || layouts.colouredLayout;
   return function(loggingEvent) {
-    consoleLog(layout(loggingEvent));
+    consoleLog(layout(loggingEvent, timezoneOffset));
   };
 }
 
@@ -14,7 +14,7 @@ function configure(config) {
   if (config.layout) {
     layout = layouts.layout(config.layout.type, config.layout);
   }
-  return consoleAppender(layout);
+  return consoleAppender(layout, config.timezoneOffset);
 }
 
 exports.appender = consoleAppender;