Create a ModelAdmin.not_editable_fields. 

If `not_editable_fields` is not empty, it is used instead of `readonly_fields` on edit model form. If it is not set, the edit model uses the `readonly_fields`.

I'm not confortable with these names, probably we need something better, but I'm sure the admin needs this feature. Django does not have it and I found some workarounds to make it work.

http://tech.fydd.in/2010/08/django-admin-making-fields-read-only.html
http://stackoverflow.com/questions/8976086/understanding-django-admin-readonly-fields

It is necessary the `fields` value to order fields. I want some fields readonly on edit but not on add.

If anyone has better names or a better approach, feel free to suggest, I'm not totally happy with this pull request, but it works for me.