Remove UUIDField contribute to class

The first commit update the tests 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.

Cheers!