make code prettify script run asynchronously

Closes #114

Author: raphamorim

src/index.html

@@ -963,7 +963,7 @@ <h2>JavaScript Projects</h2>
     </script>
   </div>
 
-  <script src="google-code-prettify/prettify.min.js"></script>
+  <script src="google-code-prettify/prettify.min.js" async></script>
   <script src="js/homepage.js"></script>
   <script src="js/download-data.js"></script>
   <script src="//cdnjs.cloudflare.com/ajax/libs/angular-ui-bootstrap/1.1.2/ui-bootstrap-tpls.min.js"></script>