Adapt django-taggit for django 1.5

Because of recent refactoring, django-taggit as it is will not work with django 1.5 and above. I've fixed that with the help of @akaariai, author of that refactoring. I'm sending that pull request to you, as your fork of django-taggit seem to be most active of all others. Changes are backwards incompatible, so there are two variants: create django1.5 branch or support both versions simultaneously. Please tell your thoughts and decision. 
Also, didn't you think about making your repository to be the "main" one? I think it's possible, @alex is a nice person.