[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.
Maybe it will be better to add a special method "add_log" that will add a new log?