Nicer student_index page

This improves the layout of the student_index page and adds the number of days left for evaluation.

Old:
![old](https://cloud.githubusercontent.com/assets/1781719/5564537/df59110c-8ec6-11e4-8733-1ec275370a09.PNG)

New:
![new](https://cloud.githubusercontent.com/assets/1781719/5564538/e445988e-8ec6-11e4-9b00-6f407eae38f1.PNG)

why not <htmltag>{{ days }} {%trans "days left" %} <endtag>?