Youtube example is now included in main page

Youtube example page is complete and ready to be displayed in the main page. You can simply uncomment the related block in the site index when you decide to show it to users. I also added a config block in `base.tmpl` so if any additional configuration (i.e. `TogetherJSConfig_youtube = true`) is necessary for an example application, it can be included in this block.

In terms of documentation, I am not so sure what more information I should introduce. I think emphasizing how youtube synchronization *just works* with TogetherJS without any extra work done by the user might be okay to include. Let me know.

Also, the youtube module fix (a pull request right before this) should be applied first in order for this example to work without errors.