Selection feature

This adds the functionality to rate applications, which was previously in the campaign app.

* It adds a new role "reviewer"
* "reviewer"'s can few and rate applications
* admins can add roles to users now

The screens look like as follows:
**Overview**:
![screen shot 2014-05-03 at 10 42 23](https://cloud.githubusercontent.com/assets/7757/2869976/774ef114-d29f-11e3-9a79-9ffb746d2384.png)

**Application Screen**:
![screen shot 2014-05-03 at 10 42 42](https://cloud.githubusercontent.com/assets/7757/2869978/8bf12f1a-d29f-11e3-9cd7-1ee88bad1ae1.png)