fixes bug 1072460 - Audit trail for all changes in admin

@AdrianGaudebert r? 
cc @lonnen 

I'm sorry for the size of this. In hindsight I should have made a PR just for adding the log entries and then make another PR for viewing the events. However, I think like this you'll be better able to test it and get a feel for what it does. 

To test, check out the branch, run `./manage.py syncdb`, `peep install -r requirements.txt` and then go to the admin. Edit some groups and users. And possibly some super search fields. Then go to `/admin/events` and they should appear there. 
Just out of curiosity, why do you write this instead of ``event = Log.objects.all()[0]``?