Implement files_at_path, directories_at_path, and stringify + write tests

How did you run the tests? I checked out this branch, and I can run the tests without Factory Girl errors (although I'm getting other errors) with the original setup (i.e., without the addition of these two lines). In fact, if I run with the `ENV['RAILS_ENV'] ||= 'test'` line deleted, I get Factory Girl errors.