2.4.0: Merge pull request #34 from anushr/xml-api

Migrate to AIM and CIM XML API, from the original (positional parameter?) API.

This affects both the AIM and CIM APIs though should have no functional effect. It does move forward to the newer API that Authorize.Net recommend is used.

However, do test this release in UAT/pre-production thoroughly before deploying to production. A lot of changed code is incorporated in this release, so some edge-case bugs may have got past us. If you find any, we will aim to fix and add new tests quickly.

Many thanks to all involved. This change has been a long time in the making.

See PR #34 for more details of the changes