Style tweaks

Using http://mockups.openstax.org/uxploration-student-view-3/index.html#p=question_2a as a rough template:

- add font-awesome
- add the orange color for buttons
- add icons for different types of steps
  - add icon for task with just 1 step
- refactor the Question component into a mixin
- change `<input>` to `<textarea>` for free-response input

Task list with icons:
![image](https://cloud.githubusercontent.com/assets/253202/5036866/48919b6a-6b51-11e4-8b44-4a2fd31bff07.png)

Multistep Assignment with icons:

![image](https://cloud.githubusercontent.com/assets/253202/5036894/771f96f8-6b51-11e4-9d3a-5ffeb3a5a40e.png)