Ultra-fast and low-CPU OS X file watching

I have added [`FSEvents`](http://en.wikipedia.org/wiki/FSEvents) support to Chokidar.

This means on OS X there finally will be no `stat` polling and no extensive CPU usage on older systems.

I'm curious how this would behave on real systems. So far in Brunch, it worked cool for me.

@es128 please test `fs-events` branch of brunch, it has all needed stuff.