[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 integration was not working despite
of the proper configuration.

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: [1]

[1]: https://github.com/imathis/octopress/blob/master/.themes/classic/source/_includes/after_footer.html
Merge pull request #66 from fabiendem/fix_after_footer

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