Merge: Custom serializer

This is an old attempt to add the possibility to have a custom serializer. The idea is that a same set of objects can be serialized in different ways if different serializers are used.

Most of what is needed to do that is already present; the main contribution of this PR is the example that tries to propose a way to do that. The example focuses on 3 simples needs:

* inject a phantom attribute.
  when serializing, the custom serializer inject new attributes.
* hide a normally serialized attribute.
  when serializing, the custom serializer hides some specific attributes.
* replace a full business object.
  instead of serializing an attribute, the custom serializer use a different representation

Pull-Request: #2426
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>