Add event emitters for events related to workers (spawn, died)

[panthershark]

Add event emitters which emit worker events via the mixdown object.

Usage

mixdown.on('worker-spawn', function(worker) {
  // do something with the new child.
});

mixdown.on('worker-ready', function(worker) {
  // do something now that the new child is ready to take requests (e.g. init is complete)
});

mixdown.on('worker-died', function(worker) {
  // do something about the death of the worker.
});