Consider use of DPSPXPost interface

James McGlinn's DPSPXPost interface is an easily re-useable interface to DPS.

It's also fully GPL, so it can be incorporated into this module cleanly. It is dependent on PEAR::HTTP_Request (currently we are dependent on cURL instead - the PEAR code could be bundled with this module to support servers with no cURL availability. Notes on DPSPXPost indicate that it's not currently PHP4 compatible, however.

http://james.mcglinn.org/2006/10/easy-dps-payment-express-interface-for-...

Having this interface available would simplify implementation of the following tickets:

• #561584: Implement Auth/Complete payment
• #561586: Implement Token Billing
• #561592: Implement Refund transactions