Slim down course model

For #9
Add course instance model

Needs more complete tests before being finished
Merge branch 'master' of github.com:theodi/odi_content_models into course-editions

Conflicts:
	app/decorators/models/artefact_decorator.rb
	lib/odi_content_models/test_helpers/factories.rb
Merge pull request #20 from theodi/course-editions

Course editions