Consistent headers

This makes all headers consistent with their corresponding link in the ToC. Primarily, it removes all mentions of "GitHub" and "API" in headers.

For example, before:

![screen shot 2013-11-22 at 21 50 31](https://f.cloud.github.com/assets/211284/1604146/d1169b32-53b7-11e3-89cb-0979c855905f.png)

After:

![screen shot 2013-11-22 at 21 51 00](https://f.cloud.github.com/assets/211284/1604147/d539f9d4-53b7-11e3-95cc-644432220d42.png)