#4 fixed the positions edit page throws 500

Fixed the positions edit page throws 500.
Add tests.
![zrzut ekranu z 2014-08-11 11 45 26](https://cloud.githubusercontent.com/assets/6718923/3875285/87f775a0-214f-11e4-89d3-9bca22c0501f.png)

hmm i think that only starts_at is needed as sorting parameter. it gives same results like `positions.select.sort_by! { |p| [p.user.first_name, p.user.last_name, p.starts_at] }`.