Using the XML API for AIM gateway

Moves the AIM gateway over to the new and improved XML API. 

I tried to leave most unit tests untouched to ensure the least amount of potential breakage, but some had to be tweaked since this was a significant overhaul. 

If anyone has some integration tests using this gateway, please do run them against this PR since I have none (my app is still not integrated with Authorize.NET).

As part of this change, the SIM classes have been completely decoupled from the AIM classes; they used to share some functionality earlier, but that's no longer possible since the endpoints and data format are now very different.