Add git sha to index for easy verification of builds.

Author: logic

index.html

@@ -2,6 +2,7 @@
 layout: default
 title: "Beware of programmers carrying screwdrivers"
 ---
+<!-- Build: {{ site.github.build_revision }} -->
 {% for post in paginator.posts %}
 <h1 class="postTitle"><a href="{{ post.url }}">{{ post.title }}</a></h1>
 <div class="post">