Integrate Django REST Framework's viewsets and routers

This branch extends D-A-E to automatically create a router from model slug and viewset pairs specified in the `{{ app_name }}.api.API_URL_VIEWSETS` dictionary. To enable backward compatibility, if importing the app's api module fails or if the `API_URL_VIEWSETS` dictionary is not present, the original autodiscover function is used.

I also converted the `README` file to `README.md` for easier formatting and readability.