group results by contributors

This groups the contributors on the result page and thereby improves the readability for courses with many contributors.

![new](https://cloud.githubusercontent.com/assets/1781719/3787709/609399b6-1a2f-11e4-809f-6cbb16f1fc31.PNG)

could you ```
if not section.results: 
    continue
```?

and ``` if section.contributor not in contributor_sections:```