Add creative work model

For #9
Merge branch 'master' of github.com:theodi/odi_content_models into creative-work-editions

Conflicts:
	app/decorators/models/artefact_decorator.rb
	lib/odi_content_models/test_helpers/factories.rb
Remove duplicate factory
Merge pull request #17 from theodi/creative-work-editions

Add creative work model