Changed most of the plain-text blocks to a use a different font

- Added a @sans-serif-family var.
- Kept number-based texts on serif/times

![screen shot 2014-11-25 at 4 20 29 pm](https://cloud.githubusercontent.com/assets/57929/5186571/0283dc6a-74bf-11e4-901a-0e4102fee561.png)

@nicpottier Just so you can easily preview/test what it would look like with a different font.