OpenID / week 5

1. finally completely fixed `next` redirect after login,
2. merged the `DateField` pull request and added birthday for it,
3. added initial support for Attribute Exchange (used e.g. by stackoverflow),
4. added storing preferences (chosen user details) in the database and 'Always allow this site' option.

To test:

    ./manage.py runserver 0.0.0.0:8000

and then go to e.g. http://www.wasab.dk/morten/2007/11/openid/ and log in with:

    http://<your-ip>:8000/id/<your-x-man>