Add cluster support

+ Number of workers specified in new config var 'worker' located in config/env/all.js, default is 1.
+ In order to handle debugging, implemented workaround discussed in comment: [joyent/node#5318](https://github.com/joyent/node/issues/5318#issuecomment-16702390)
If the debug worker dies, it needs to spawn a new debug thread too. I did this on one of my projects with code like this: 

var spawnWorker = function(deadWorker, debugPort) {
    if (!debug)
      return cluster.fork();
    
    var workerDebugPort = deadWorker === null ? debugPort : deadWorker.debugPort;
    cluster.settings.execArgv.push('--debug=' + workerDebugPort);
    var newWorker = cluster.fork();
    _.assign(newWorker, {debugPort: workerDebugPort});
    cluster.settings.execArgv.pop();
    
    return newWorker;
  }