New feature: logging object actions in django admin

[ooppsss60]

It can be useful when there are several admins on a project.

[crccheck]

neat idea. It needs:

1. a test that shows the message is created and what the message looks like
2. something in the README

I'm a bit concerned about models that might not have a pk (I've run into this importing an existing schema into a Django project) and I think Django is going to support composite keys soon. I'm rusty and just woke up so I may be hallucinating