Use `fsw` on OS X for file system watching

[fsw](https://github.com/emcrisostomo/fsw) is a minimal wrapper around a few different file system notifications. In this case we are using it to wrap the OS X File System Events API.

This greatly simplifies install as `fsw` is a brew installable dependency. On linux, we can continue with `inotify` or we could also use `fsw` on linux as it justs wraps `inotify`.