Add space above h2 and h3 elements

This is certainly a cosmetic change, but one that helps me read the spec. Currently it's difficult to see `h3` headlines, especially after sections that end in a table.

This change simply updates both the `h2` and `h3` elements to have 50 pixels of margin above, rather than 10 and 15, respectively.

![screen shot 2014-12-03 at 1 15 50 pm](https://cloud.githubusercontent.com/assets/46891/5289027/480b39f0-7aef-11e4-8593-32f54a9761c0.png)