It now takes the shipping charge rather than the method. This makes
sense as the shipping charge has normally been calculated before we need
the order total so there is no point in calculating it twice.

To support multi-currency sites.  This commit also made the order totals
a mandatory input for creating an order, which meant quite a few tests
had to change to support the new API.

It now returns a OrderTotal instance which has price fields with- and
without tax.