Fine tune slide title styling

Cleans up the overly bold headings of the chapter covers and the `h1` and `h2` titles of slides.

![screen shot 2014-06-02 at 11 20 20 pm](https://cloud.githubusercontent.com/assets/352082/3148484/2be39d9a-ea61-11e3-9813-636b9dc9e86c.png)
![screen shot 2014-06-02 at 11 20 22 pm](https://cloud.githubusercontent.com/assets/352082/3148486/2beb6e9e-ea61-11e3-8645-37db6d7109e4.png)