[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
This modules need sale_cancel_picking that you can find on this PR : https://github.com/OCA/sale-workflow/pull/69 
Do not use \ and even less inside strings.
BTW the line is not translated. What is `order.add_logs()`?
```
log_line = ("<p>Number of deleted lines in Purchase "
            "Orders: %d</p>" % len(po_line_ids))