Support product code in old-style basket

Details on Issue #107

The filters for the old-style basket product name have been fixed to allow a wider range of allowed characters. This include square brackets [] which have a special meaning to upstream Sage products to help account reconciliation.

The extended Item has an extended interface to include productCode, which is automatically inserted into the basket line name, in square brackets, as a convenience.

The productCode is not inserted into the XML basket at this time, and it probably should be. However, Sage/Sage Pay does not support the same upstream product reconciliation at this time, so it is less helpful.

There should be no BC breaks, but the change to the extended Item interface makes it prudent to make this a minor release.