Use HTML5 article tag for blog post summaries

See: https://developer.mozilla.org/en/docs/Web/HTML/Element/article

layouts/_default/summary.html

@@ -1,6 +1,6 @@
-<div class="blog-post">
+<article class="blog-post">
   <h2 class="blog-post-title"><a href="{{ .Permalink }}" title="{{ .Title }}">{{ .Title }}</a></h2>
   <p class="blog-post-meta"><time datetime="{{ .Date }}">{{ .Date.Format .Site.Params.date_format }}</time> by <a href="#">Mark</a></p>
   {{ .Summary }}
   <a href='{{ .Permalink }}'><nobr>Read more →</nobr></a>
-</div> <!-- /.blog-post -->
+</article> <!-- /.blog-post -->