Put social usernames in the _config.yml

So you don't have to edit the _includes/header.html
Repair vertical rhythm in _blog.scss
Move disqus js into _include and add shortname to _config.yml

No disqus config entry means disqus js will not be included
Add owner name and use as default author for posts if none supplied

_config.yml owner_name appears at the bottom of each page and as the
author of a blog post if no author has been stated in the post's yaml
header. Allows multiple authors for one site.
Add "author" to yaml two posts to demonstrate names in blog index