Use async Google Analytics tracking code

_layouts/default.html

@@ -48,7 +48,15 @@ <h2 id="project_tagline">{{ site.description | default: site.github.project_tagl
       </footer>
     </div>
 
-
-
+    {% if site.google_analytics %}
+      <script type="text/javascript">
+        (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', '{{ site.google_analytics }}', 'auto');
+        ga('send', 'pageview');
+      </script>
+    {% endif %}
   </body>
 </html>