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.

This is to support US-like sites where sales taxes are only known once
the customer enters their shipping address.

This requires several changes to templates to show prices exclusive of
tax when tax isn't known.

Offers are currently now applied until taxes are known as it is unclear
to me whether to use the tax-exclusive prices to determine whether
customer's meet value conditions.  This may change later.