Switch from fsnotify to a simple fspoll.

Polling is bad and eats CPU and battery, but fsnotify simply doesn't
work with OS X with many files. There is an effort to create a
filesystem notification package for standard library --
https://github.com/go-fsnotify/fsnotify -- but it currently doesn't
support OS X too, so until it's ready, let's use a very primitive
polling implementation.