Adds a basic page template

Many users moving to Jekyll from other popular blogging options (like
Wordpress) will be used to thinking in terms of pages/posts. Having a
page template like this will be convenient for those users.

lib/site_template/_layouts/page.html

@@ -0,0 +1,14 @@
+---
+layout: default
+---
+<div class="post">
+
+  <header class="post-header">
+    <h1>{{ page.title }}</h1>
+  </header>
+
+  <article class="post-content">
+  {{ content }}
+  </article>
+
+</div>