Change course reserve masthead markup to be dl instead of p.

Closes #478 

Adding the explicit width and `margin: auto` was the only way I could get the `dl` to play nicely w/ `text-align: center`.

![crez-masthead-dl](https://cloud.githubusercontent.com/assets/96776/3589742/fb7ec1d6-0c50-11e4-8acc-4fd0075795ab.png)


![crez-masthead-dl-markup](https://cloud.githubusercontent.com/assets/96776/3589744/fe643c1e-0c50-11e4-8a83-8c30b1715203.png)