Render assignment with multiple steps

This PR is to #2 so **this PR** should probably be merged 1st (but after #4 is merged).

Empty assignment (Next button is disabled):
![image](https://cloud.githubusercontent.com/assets/253202/4985207/fd45f2be-692a-11e4-8971-c7427fa61453.png)

Exercise answer filled in (Next button is enabled):
![image](https://cloud.githubusercontent.com/assets/253202/4985210/06ae2056-692b-11e4-8ade-c55c20f63283.png)

Multiple choice question (with first exercise answered):
![image](https://cloud.githubusercontent.com/assets/253202/4985245/6dbca7cc-692b-11e4-8fb1-ac0893a387a5.png)

Exercise with Math and all questions filled in:
![image](https://cloud.githubusercontent.com/assets/253202/4985262/a31573ae-692b-11e4-9edb-a963ea45e8bf.png)