Blog: Add Blog To Bitcoin.org

* Adds new page /en/blog

* Adds new feed for blog posts to /en/rss/blog.xml

* Adds "Blog" link to site footer

* Adds three back-dated blog posts to give the blog some initial
  content.

en/blog.html

@@ -0,0 +1,27 @@
+---
+# This file is licensed under the MIT License (MIT) available on
+# http://opensource.org/licenses/MIT.
+
+layout: base
+lang: en
+title: Bitcoin.org Site Blog - Bitcoin
+id: site-blog
+---
+
+<div class="home">
+
+  <h1 class="page-heading">Bitcoin.org Site Blog</h1>
+
+  <p class="summary">Discover what's new on Bitcoin.org or <a href="/en/rss/blog.xml">subscribe to the RSS feed</a></p>
+
+  <ul class="post-list">
+    {% for post in site.posts %}
+      <li>
+        <a class="post-link" href="{{ post.url | remove: '.html' }}">{{ post.title }}</a>
+        -
+        <span class="post-meta">{{ post.date | date: "%b %-d, %Y" }}</span>
+      </li>
+    {% endfor %}
+  </ul>
+
+</div>