Get order status change working

Note: I'm having trouble testing these changes using the sandbox site. I just copied over the changes from the installation in my virtualenvironment, where they work fine. This fork is different from that in my virtualenv.

I have not implemented order status change cascading to line status changes, so changing an order from 'Being processed' to 'Delivered' will still leave items in the order as 'Being processed'. Also, a warning is generated on the page, after changing the status, that shouldn't be there. 
You only want `change_order_statuses` here.