Integrate Chokidar into Watcher module

Integrate Chokidar into Watcher module instead of using setInterval.  I've smoke tested this against [broccoli-sample-app](https://github.com/broccolijs/broccoli-sample-app) and it has worked beautifully.

This resolves the sleep issue described in #144 and the CPU issue described in #72

Let me know what you think.  :))))