Add IBAN model and form fields.

This pull request adds an IBAN model and form field as per the discussion in issue #19. This code is mostly a cleaned up version of the code from django-iban with the addition of the option to use the Nordea extensions. Even thought this code is from an existing module, it's never had a comprehensive review so I'm open comments and suggestions.

I'm going to do the SWIFTBIC field as a separate pull request after this has been reviewed and merged.