Use HTML5 header tag for blog header

Makes it easier for computers to understand content by using tags
that have explicit meaning.

See: http://diveinto.html5doctor.com/semantics.html

layouts/_default/baseof.html

@@ -31,12 +31,12 @@
     </div>
   </div>
 
-  <div class="blog-header">
+  <header class="blog-header">
     <div class="container">
       <h1 class="blog-title">{{ .Site.Title | safeHTML }}</h1>
       {{ if .Site.Params.description }}<p class="lead blog-description">{{ .Site.Params.description | safeHTML }}</p>{{ end }}
     </div>
-  </div>
+  </header>
 
   <div class="container">
     <div class="row">