Replace watcher.js with broccoli-sane-watcher.

Replaces watcher.js with @krisselden's broccoli-sane-watcher 

Should resolve #72 and replace #150

On our largish ember.js project we went from consistent 25% cpu with current watcher to roughly 0% using the sane-watcher

cc: @stefanpenner @rondale-sc @krisselden