Add Fira font family (Sans and Mono) as a git submodule

Designer: Erik Spiekermann, in collaboration with Carrois Type Design
Commissioned by: Mozilla, for Firefox OS

Home Page:
http://dev.carrois.com/fira-3-1/

GitHub Project:
https://github.com/mozilla/Fira
Use 'Fira Sans' instead of 'Source Code Pro' in headings and copy
Apply "Fira Mono" to pre, code and data elements