Create Directory model.

Current spec status: 8 examples, 0 failures in 0.13926 seconds.
Set up tree relations for Directory model.

Current spec status: 33 examples, 0 failures in 0.25576 seconds.
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.
Merge pull request #1 from sleepingkingstudios/feature-directories

Create Directory model, DirectoriesController.