Added unique-constraint to slug_composed in Category model.
By making the last slash in the category slug urls part of the regexp we can omit the format()-function in the views and asure that strings from the Category.slug_composed field match nicely inside of the view.
Forgot to take out the ipdb.set_trace(). Fixed
Fixed raise 404 -> raise Http404
Added first test for a 200 on category list and refactored todos for model tests to newly created test_models.py