Map "contentinfo" role to HTML5 footer element

http://blog.paciellogroup.com/2013/02/using-wai-aria-landmarks-2013/#tablex

http://www.w3.org/TR/wai-aria/roles#contentinfo

_layouts/home.html

@@ -17,7 +17,7 @@
     <!-- Callout for the old docs link -->
     {% include old-bs-docs.html %}
 
-    <div class="container" role="contentinfo">
+    <footer class="container" role="contentinfo">
       {% include social-buttons.html %}
 
       <ul class="bs-masthead-links">
@@ -34,7 +34,7 @@
           <a href="{{ site.expo }}" onclick="_gaq.push(['_trackEvent', 'Jumbotron actions', 'Jumbotron links', 'Expo']);">Bootstrap Expo</a>
         </li>
       </ul>
-    </div>
+    </footer>
 
     <!-- JS and analytics only. -->
     {% include footer.html %}