Address burtlo#94

Attempts to fix "uninitialized constant <Module:...>::TreeContext"
failure when generating the class list nav running `yard server`.

See burtlo#94 for details.

If there are tests, I haven't figured out how to run them (see
burtlo#96 for details).

lib/templates/default/fulldoc/html/setup.rb

@@ -163,7 +163,7 @@ def generate_full_list(objects,type,options = {})
 # This method removes the namespace from the root node, generates the class list,
 # and then adds it back into the root node.
 #
-def class_list(root = Registry.root, tree = TreeContext.new)
+def class_list(root = Registry.root, tree = self.class.const_get(:TreeContext).new)
   return super unless root == Registry.root
 
   cucumber_namespace = YARD::CodeObjects::Cucumber::CUCUMBER_NAMESPACE