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/smtp.js

@@ -28,7 +28,7 @@ function smtpAppender(config, layout) {
       var firstEvent = logEventBuffer[0];
       var body = "";
       while (logEventBuffer.length > 0) {
-        body += layout(logEventBuffer.shift()) + "\n";
+        body += layout(logEventBuffer.shift(), config.timezoneOffset) + "\n";
       }
 
       var msg = {