Fix katiefenn/parker#30.

This uses https://www.npmjs.org/package/graceful-fs as a drop-in
replacement for the fs module. This will of course have a minor
performance impact, but it should make Parker more resilient when
processing large numbers of stylesheets.

See also:
http://stackoverflow.com/a/15934766/349353
https://github.com/joyent/node/issues/5401#issuecomment-17399695