Fix disqusShortname conditional

Fixes #43

layouts/_default/single.html

@@ -9,7 +9,7 @@
 			      {{ .Content }}
 			</div>
 
-			{{ if and (isset .Site.Params "disqusShortname") (ne .Site.Params.disqusShortname "") }}
+			{{ if .Site.Params.disqusShortname }}
 				<h2>Comments</h2>
 				{{ partial "disqus" . }}
 			{{ end }}