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/layouts.js

@@ -71,11 +71,11 @@ function colorize (str, style) {
   return colorizeStart(style) + str + colorizeEnd(style);
 }
 
-function timestampLevelAndCategory(loggingEvent, colour) {
+function timestampLevelAndCategory(loggingEvent, colour, timezoneOffest) {
   var output = colorize(
     formatLogData(
       '[%s] [%s] %s - '
-      , dateFormat.asString(loggingEvent.startTime)
+      , dateFormat.asString(loggingEvent.startTime, timezoneOffest)
       , loggingEvent.level
       , loggingEvent.categoryName
     )
@@ -93,18 +93,19 @@ function timestampLevelAndCategory(loggingEvent, colour) {
  *
  * @author Stephan Strittmatter
  */
-function basicLayout (loggingEvent) {
-  return timestampLevelAndCategory(loggingEvent) + formatLogData(loggingEvent.data);
+function basicLayout (loggingEvent, timezoneOffset) {
+  return timestampLevelAndCategory(loggingEvent, undefined, timezoneOffset) + formatLogData(loggingEvent.data);
 }
 
 /**
  * colouredLayout - taken from masylum's fork.
  * same as basicLayout, but with colours.
  */
-function colouredLayout (loggingEvent) {
+function colouredLayout (loggingEvent, timezoneOffset) {
   return timestampLevelAndCategory(
     loggingEvent,
-    colours[loggingEvent.level.toString()]
+    colours[loggingEvent.level.toString()],
+    timezoneOffset
   ) + formatLogData(loggingEvent.data);
 }