Clustered appender for log4js.

+ lib/appenders/clustered.js
+ test/clusteredAppender-test.js

Instead os using sockets (like multiprocess) or dead and unmaintained hook.io, Clustered appender
uses process.send(message) / worker.on('message', callback) mechanisms for transporting data
between worker processes and master logger.

Master logger takes an "appenders" array of actual appenders that are triggered when worker appenders send some data.
This guarantees sequential writes to appenders, so the log messages are not mixed in single lines of log.
Merge pull request #149 from mkielar/master

Clustered appender for log4js.