Set up testing framework for core models

- Moved models to new app called 'core' (makes testing of models easier)

- Set up core.tests.recipes using model_mommy to generate mock versions
of models via recipes.[model].make()

- Set up tests in core.tests.test_models

- added 'discover_runner' to apps to improve Django's default testing
behaviors

- some notes in models.py re: related fields