Create DirectoriesController.

- Add #show action to find directories matching routing path.
- Remove ScaffoldController, since new DirectoriesController handles
root path.
- Update Directory::find_by_ancestry to return array of found
directories upon success, raise Directory::NotFoundError on failure.
- Create Directory::NotFoundError class with syntax similar to
Mongoid::DocumentNotFound.

Current spec status: 39 examples, 0 failures in 0.30701 seconds.