Moving all script tags into head with defer attribute

As discussed by email, it's best to use `defer` attribute on script tags and put them into the `head` section to boost performances (they're downloaded in parallel and don't halt HTML parsing). All modified examples have been tested, it was easy as you use the exact content of files through iframes! The tests for x-tab-panel with mocha works too. I modified the **public/pages/sites.html** content to reflect the position change of the script into the head and not the body anymore. Last point, I just modified the examples, not the core code of your website but I can upgrade it if you want too.