[website] Add Google Analytics.

_layouts/default.html

@@ -70,6 +70,14 @@
       <div class="right">&copy; 2013 Facebook Inc.</div>
     </footer>
   </div>
+  <script>
+    (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
+    (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
+    m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
+    })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
+    ga('create', 'UA-41298772-1', 'facebook.github.io');
+    ga('send', 'pageview');
+  </script>
 </body>
 </html>