fix bug 844137 - Add fonts from gaia

New "fonts.css" file.

To spot-check:

1.  Go to homepage
2.  In console:  jQuery("h1").css("font-family", "MozTT");   
3.  (watch it change)
4.  In console:  jQuery("h1").css("font-weight", "lighter");   
5.  (watch it change)