Edit _layouts/post.html to remove disqus if unused

Now, leaving out a disqus name in _config.yml will have the same effect
as turning comments off on each individual page.

_layouts/post.html

@@ -48,7 +48,7 @@ <h2><span class="entry-date date published"><time datetime="{{ page.date | date_
         {% if page.share != false %}{% include social-share.html %}{% endif %}
       </footer>
     </div><!-- /.entry-content -->
-    {% if page.comments != false %}<section id="disqus_thread"></section><!-- /#disqus_thread -->{% endif %}
+    {% if page.comments != false and site.disqus_shortname %}<section id="disqus_thread"></section><!-- /#disqus_thread -->{% endif %}
     {% if site.related_posts.size > 0 %}{% include read-more.html %}{% endif %}
   </article>
 </div><!-- /#main -->