add date to post (#18)

Author: fu5ha

_layouts/post.html

@@ -2,6 +2,7 @@
 layout: default
 ---
 <h1>{{ page.title }}</h1>
+<h3><span>{{ page.date | date_to_string }}<span></h3>
 
 <div class="content">
 	<div class="post">{{ content }}</div>