External Referential Refactoring

The current implementation of the multi-backend support in openopbject-extension/base_external_referential + e-commerce-addons/base_sale_multichannel is very awkward as it only uses openobject inheritance for the implementation, and different backends need a different implementation. The current solution uses decorators to handle this.

A better way is to decouple the code of the referential from the code responsible for the communication with the backend using the Bridge design pattern. In that way, the information needed to connect with the backend is stored in the referential, as well as information used to find the correct implementation of a Connector (backend specific) and a Mapping (backend specific). These objects do not live in the OpenERP database, and can be overriden on a per instance basis.