Add a Youtube widget

There are still some commits to come to get it to work with our new development environment,
namely that the plugin is being loaded from outside of the ckeditor plugins folder, such as updating `test.html`.
As far as I can tell this is verbose so that we can put in an iframe.

This jquery plugin should really just bind to an empty div and we keep control of what goes inside it. We could provide a config for intialising the plugin which determines whether we use an iframe but we can do that later. e.g.

    <div id="#youtube-search-form">

    $('#youtube-search-form').youtubeSearchPlugin(config)