Added tests for 404 on invalid cat_id or cat_slug for child leaf with no siblings
Refactoring set creation related TODOS to test_models.py
Dropping unnecessary set creation in testcase for category 404
Possible null values for description and default order value of int(1)
Added first tests for the Category model, refactored create_category() function from test_views.py to helpers.py (code largely borrowed from the django filer test package)