[Admin] Fully supports related fields (fk, m2m, 1to1, genericfk) to chil...

...d models.

This fixes #46, #47, and perhaps #57.
This also fixes some issues with grappelli related lookups.

WARNING: This changes the syntax of the admin integration.

There was no option but to change the syntax.  I tried keeping the same syntax and it was not working because while the admin site registry was iterated by Django, the size of the registry changed because of the lazy setup.  The only solution was to stick to the classical Django syntax.