devilry_gradingsystem.models: Turned the skeleton into a usable model.
devilry_gradingsystem: Created initial migration.

We have already pushed up this without an initial migration, so
if you have created the testdb, you have to do the following:

	$ bin/django_dev.py sqlclear devilry_gradingsystem | sqlite3 db.sqlite3
	$ bin/django_dev.py migrate devilry_gradingsystem
apps.core.models.tests.assignment: Updated now that the grading setup

is valid by default.