Improve H2 and H3 header style

The all-uppercase H3s have been annoying me for a while. This PR changes the styling of H2 and H3 to be more standard, using the sizes 24px and 18px.

Before:

![before](https://cloud.githubusercontent.com/assets/6269/2753189/cab25620-c92b-11e3-896e-5a4b23667f5c.png)

After:

![after](https://cloud.githubusercontent.com/assets/6269/2753188/ca87fd76-c92b-11e3-93e9-7b1fba02dc31.png)