[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
You should add a berfore log message. Something like "can not delete the purchase line for the product XXXX as the purchase order YYYY is in the state ZZZZZ