Display the name of the 'Last updated by' user

- it displays 'Last updated... by [Person]' when the person is *not* the system user (Version.public_user0

- see 'Last updated ... by Toby' (in attachment, lower right of page)
![screen_shot_last_update](https://cloud.githubusercontent.com/assets/5799/4886165/fb662cea-6379-11e4-8843-017b5cb728f8.png)

Should we maybe use i18n date time formatting, so the helpers are not responsible for it.

```ruby
l(obj.updated_at)
```

In `config/locales/en.yml` we can then set a default format:

```yaml
en:
  time:
    formats:
      default: "%d %b %Y %H:%M"
```