[IMP] orm: remember Model._original_module when constructing inheritance

ORM Models already have a _module attribute that contains the
name of the module that declared this class, however
sometimes we also need the name of the module that
declared this model the first time.
This will be stored in _original_module and is the
name of the module to which the first parent with 
the same _name belongs to.

bzr revid: odo@openerp.com-20111004204705-8z9o70n1ynpvng3i