[Fix] Include the after_footer.html in the default layout

after_footer.html was never called, which means that some library's specific scripts were never included.

Because of this, the Disqus feature was not working despite of the proper configuration.

I have put the call just before the closing of the body, as after_footer normally includes `<script></script>` calls.

The following scripts are included by default:

```
{% include disqus.html %}
{% include facebook_like.html %}
{% include google_plus_one.html %}
{% include twitter_sharing.html %}
{% include custom/after_footer.html %}
```

See also the classic theme: 
* [Classic after_footer.html][1]
* [Classic default.html][2]

[1]: https://github.com/imathis/octopress/blob/master/.themes/classic/source/_includes/after_footer.html
[2]: https://github.com/imathis/octopress/blob/master/.themes/classic/source/_layouts/default.html