ordered_tree: make OrderedTree a subclass of Collection and implements missing methods

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
ordered_tree: add some tests

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
Merge: ordered_tree: make OrderedTree implement Collection

OrderedTree now implement major Collection services

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

Pull-Request: #524
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>