Refactor code; fix not showing anonymous feedback in the admin

I did some refactoring, fixed an outdated url tag and then fixed issue #10, displaying anonymous feedback in the admin if enabled. If there is anonymous feedback it says it's from the user "anon".

Note: I made a local change to a template name, see [feedback/views.py](https://github.com/hph/django-feedback/blob/53dd957504d9f3dea87c068a5959d9f804d821b2/feedback/views.py#L8) line 8, changed in 53dd957, that should probably not be merged. Just leave this change out.