add an Exercise Task supporting multiple formats

Satisfies <https://www.pivotaltracker.com/n/projects/1156756/stories/79368948> and <https://www.pivotaltracker.com/n/projects/1156756/stories/80836772> but keeping it WIP because I would like to:

- [x] add code for detecting which item was selected (using [flux-react](https://github.com/christianalfoni/flux-react) )

![image](https://cloud.githubusercontent.com/assets/253202/4809360/ec1a9ab0-5eaa-11e4-94b5-07118e5c616c.png)

![image](https://cloud.githubusercontent.com/assets/253202/4809381/113beaf6-5eab-11e4-8f7f-de2aab9473c1.png)