Refactor header markup

- use anchor instead of h1 for site title, for semantic accuracy, and
because post h1’s should be the post title
- implement nav wrapper for nav links
- use div.wrap for design structure

lib/site_template/_includes/header.html

@@ -1,3 +1,14 @@
-  <div class="header">
+<header class="site-header">
-    <h1 class="title"><a href="/">{{ site.name }}</a></h1>
+
-  </div>
+  <div class="wrap">
+
+    <a class="site-title" href="/">{{ site.name }}</a>
+
+    <nav class="site-nav">
+      <a class="page-link" href="/about">About</a>
+      <a class="page-link" href="/projects">Projects</a>
+    </nav>
+
+  </div>
+
+</header>