Bulk order status update

Second attempt - this one is a cleaner pull request :-). 

This pull request adds capability to update order statuses in bulk. I have tested it manually, but not written any unit or functional tests yet (I'm still trying to figure out how to do that, trying to understand the existing tests first).

Note:

1. This does not cascade the order status changes down to the line items
2. It always shows a warning on the page on reload after successful submission, saying that the new status for the last order status changed is not valid for that order, even though it actually is valid. 

Otherwise, this seems to work just fine.

UPDATE:

Problem 2 above is fixed, and I added a test to test this new functionality.
All this is already done by ``BulkEditMixin``. You should be able to add a function like this: ``def change_order_statuses(self, request, orders)`` and if it should be called if the template is setup correctly. Look at how it works for ``download_selected_orders``.