[IMP] sale_stock Show the button 'view delivery order' as soon as a picking exists

The button should appear no matter the state of the sale order, to be consistent with the rest of the application.
This makes it much faster for the sales person to find out about orders in shipping exceptions or already processed.
Fixes #256
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32