Minor type tweaks

Amazing work on the site, I was just checking out the updates and made some minor tweaks along the way. Feel free to take as many or as few as you wish.

- Expands upon fallback fonts (Avenir for OS X and iOS, adds Typekit Sync names, line height when falling back to Georgia)
- Carries down some `font-feature-settings` manually
- Makes the header lowercase for the _st_ ligature:

![screen shot 2014-12-15 at 10 54 39](https://cloud.githubusercontent.com/assets/1581276/5441903/6e818356-8449-11e4-804b-61e5a5d427f5.png)