Improve default rpc from fields registration
Linked to the comment in [1] we should probably review all the model/ directory for similar cases where isinstance() and hasattr() are used to find the field type where probably responsability should be delegated to the field.
This would bring more flexibility to adding new field types as well as simplify the code on model*.py.
[1] http://codereview.tryton.org/13401002/diff/1/trytond/model/model.py#newcode108