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. 
I think you can add ``delete`` here as well. Unless you only want to show URLs that can be clicked to lead to the same action page? (While testing I was happy to have a quick way to see what field was impacted by just hovering the URL link. )