Updates to backend:
- Created API resources for Grammar and Content models.
- Populated Grammar models. Fixture data available in app/fixtures/db.json.
- Added to settings.py to resolve test runner warning.
- Some cleanup to urls.py.
- Upgraded to latest version of markdown (activate virtual env to update requirements).

Progress on #25.
Merge pull request #26 from OpenPhilology/api

Updates to backend.