add author, correct url to config.yml, fix social

* _config.yml
  - added site.author so author links at the bottom of posts point to
    the about page
  - changed site.url from apex domain to www. This is to better reflect
    the canonical url for the site

* _layouts/page and _layous/post
  - modified social sharing to append site.url to arguments, since the
    links were sending just /path/to/page.ext as the shared page's url