[ADD] Modules sale_cancel_purchase and sale_purchase_link

OpenERP 7.0 Module to cancel a sale order and by the same time, related purchase order lines. That simply delete the lines that are linked to the sale order thanks to sale_purchase_link.

sale_cancel_purchase depends on sale_purchase_link
Not sure that using the orgin is the best way. Can you test with kit? Maybe we can have a better solution by looking to the outgoing move which is linked to the sale order?