[FIX] purchase: Do not allow to delete a purchase order line which is validated

This restriction behavior copied from the sale.order model
Moreover, the purchase.order lines state were not set to cancel when the purchase order was cancelled.
This is now the case, this behavior is also coped from the sale.order model
When the purchase order is reset to draft, we also reset the order lines state to draft
[FIX] replace .bzrignore by .gitignore, as we are now working with git
Subtree merged in odoo/7.0