Fix #227 .less files not being auto-recompiled

Quick googling turns up hundreds of wrappers around fs.watch() that
watch recursively, as well as doing lots of unnecessary stuff like
allowing globbing paths, all of which are also hundreds of lines long.

The fs.readdir() trick is from isaacs' filewatcherthing:
https://github.com/isaacs/filewatcherthing/commit/740c76db4bcfd96bb087eca7f47d02828df2788d

I'm not sure why isaacs guarded against '.' and '..', fs.readdir()
supposedly (and empirically) excludes them:
http://nodejs.org/api/fs.html#fs_fs_readdir_path_callback