Allow stockrecord currency to be set in test factory
Adjust core pricing policies to have a currency attribute
Add currency field to basket line

Plus provide a currency property on the basket itself.
Rename migration

To solve dupliate numbering issue after merge from master.
Store currency with order

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.