Fix BlahTeX/PNG Path

Dunno why Ari tolerated this
up till now.

lib/url_generator.rb

@@ -7,6 +7,8 @@ class AbstractUrlGenerator
     @controller = controller
   end
 
+  attr :controller
+
   # Create a link for the given page (or file) name and link text based
   # on the render mode in options and whether the page (file) exists
   # in the web.