Use Google Fonts for Fira Sans

Previously, code.cdn.mozilla.net was used, which for some reason does
not set expiration time which prevents caching the fonts. Also, Google
Fonts now supports WOFF2 (smaller file sizes) and Fira still has it's
awesome ligatures.

Oh, and we load the fonts with async JS now. Async font loading is cool.

Author: killercup

_includes/head.html

@@ -8,6 +8,13 @@
     <meta name="description" content="{{ site.description }}"/>
 
     <link rel="stylesheet" href="{{ "/styles.css" | prepend: site.baseurl }}"/>
-    <link rel="stylesheet" href="//code.cdn.mozilla.net/fonts/fira.css" />
+    
+    <script type="text/javascript">
+      WebFontConfig = {
+        google: { families: [ 'Fira+Sans:400,500,400italic:latin' ] }
+      };
+    </script>
+    <script src="//ajax.googleapis.com/ajax/libs/webfont/1/webfont.js" async></script>
+
     <link rel="canonical" href="{{ page.url | replace:'index.html','' | prepend: site.baseurl | prepend: site.url | replace:'//','/' }}"/>
 </head>