Add generator tag to Atom feed.

generator is an optional element, but let's give credit where it's due.
Besides, it can be helpful to feed consumers to know what tool generated the file.

.themes/classic/source/atom.xml

@@ -15,6 +15,7 @@ layout: nil
       <email>{{ site.email | xml_escape }}</email>
     {% endif %}
   </author>
+  <generator uri="http://octopress.org/">Octopress</generator>
 
   {% for post in site.posts limit: 20 %}
   <entry>