Add Lato typeface

Helvetica is cool, but [Lato](https://www.google.com/fonts/specimen/Lato) is cooler. It's also open source, and freely available to anyone who wants to use it. So let's embed it in our app! Yay! :tada: 

This branch adds basic support for using Lato everywhere. I'll add better typography rules in another branch.

cc @jkleinsc