- Support DAGs.
 - Add is_leaf, is_parent, is_branch, and is_root.
 - Tree::Visitor
 - Come up with a better name for attach_before and 
   attach_after.
 - Support multiple columns for ordering.
 - Declare both the parent column and the position column 
   in one call.
 - Add an ancestors() and descendants() method with 
   support for resultset cacheing.