Fix hook importing

Hook importing is currently broken for me. As @Rarst noted in https://github.com/rmccue/WP-Parser/issues/55#issuecomment-36270559, a change was made from `enterNode()` to `leaveNode()` upstream. Because of this, the reflector for a node doesn't get set until the node is exited. Since the hooks are added to the reflectors, they now have to be added in `leaveNode()`, since only after calling `parent::leaveNode()` do the reflectors become available.