Better handling of face part parameters

* If the requested face part isn't there, we use the 'first' one we have
  (ordered by the filesystem call)
* If the requested face part is 'x', we don't show that part
* Significant refactor of all filesystem stuff into a helper class
Whaaargarbl

Fix typo.
Remove unused helper method

We moved all the filesystem stuff to the ImageFiles module.
Merge pull request #23 from adorableio/better_face_parts

Better handling of face part parameters