Remove UUIDField's `contribute_to_class` method

The first commit updates a test to trigger issue #329.

The second commit is the fix. 

This is a tricky one because merging this will break inline editing in the admin because of Django bug 13696

However I believe it's the right thing to do. Instead of implementing a bogus workaround in django_extensions, we should try to get the inline editing bug fixed in Django itself and in the meantime implement workarounds in our projects by overriding admin templates. Note that this pull request doesn't break any existing test.

Cheers!