Fixes for newer django

- Fixed use of ADMIN_MEDIA_PREFIX, since it was deprecated.
- Added logic to dynamically add popup widgets to all necessary fields in a form.
- Added a form to inherit from, which automatically adds popup widgets to all necessary fields.
- Added support for MultipleSelect (not tested).
- Improved code to obtain the model from its name, and to obtain the normalized model's name.
- Improved popup template to use a verbose field name.
- Modularized code.
- Some PEP8 fixes.