Include tag/category links on wiki pages

e.g. Pages with the 'Cookbook' tag should have a link to
http://biopython.org/wiki/Category%3ACookbook

Note we have to replace spaces in the tags with underscores
so that e.g. 'Wiki Documentation' will link to the URL
http://peterjc.github.io/wiki/Category%3AWiki_Documentation

This is provisionally done as a bullet list above the page
title, and would benefit from some CSS styling, but is enough
to close #104.

Thank you to @Gasta88 for finding a related example which was
very helpful: http://charliepark.org/tags-in-jekyll/

_layouts/wiki.html

@@ -3,6 +3,9 @@
 ---
 
 <div class="page">
+  <ul class="tag_list_in_post">
+    {% for tag in page.tags %}<li class="inline tag_list_item"><a class="tag_list_link" href="/wiki/Category%3A{{ tag | replace:' ','_' }}">{{ tag }}</a></li>{% endfor %}
+  </ul>
   <h1 class="page-title">{{ page.title }}</h1>
   {{ content }}
 </div>