Extends Purchase module:
- Adds a 'Responsible' field to purchase orders:
This is a user who will follow up the order. This users replaces
the creator in the order's mail messages followers list, and in the
create_uid ORM field. His user's contact info is printed on
purchases orders as 'Referent'.
- A filter w.r.t. the mail sellers is placed on the products field of a
purchase order.
- Allow inverting the Purchase Order Reference on the invoice lines.