Restore async on demo scripts

Author: eligrey

demo/index.xhtml

@@ -49,9 +49,9 @@
             <input type="submit" value="Save"/>
         </form>
     </section>
-    <script src="https://raw.github.com/eligrey/Blob.js/master/Blob.min.js"></script>
-    <script src="https://raw.github.com/eligrey/canvas-toBlob.js/master/canvas-toBlob.js"></script>
-    <script src="https://raw.github.com/eligrey/FileSaver.js/master/FileSaver.js"></script>
-    <script src="https://raw.github.com/eligrey/FileSaver.js/master/demo/demo.js"></script>
+    <script async="" src="https://raw.github.com/eligrey/Blob.js/master/Blob.min.js"/>
+    <script async="" src="https://raw.github.com/eligrey/canvas-toBlob.js/master/canvas-toBlob.js"/>
+    <script async="" src="https://raw.github.com/eligrey/FileSaver.js/master/FileSaver.js"/>
+    <script async="" src="https://raw.github.com/eligrey/FileSaver.js/master/demo/demo.js"/>
 </body>
 </html>