Implements model save options, issue #110

Original pull request #110, resubmitting for merge to the develop branch.

This PR implements the 'Save', 'Save and continue editing', and 'Save and add another' options on the model create page. Tests for the appropriate redirects are included.

Note that the original admin2 behaviour of redirecting 'Save' to the model 'detail' page is unchanged, even though this differs from the contrib.admin behaviour of redirecting to the model index page. It's unclear if this is a feature or a bug, so I left it alone.