Maintain separate credentials for debug and active PayPal accounts

The e-mail associated with the business and live accounts from PayPal are intentionally different. The servers are also different. Changing both in the configuration page is tedious and, if it has been a long time since the site was maintained, the debug account e-mail may be lost or forgotten.

The module should maintain both sets of credentials, and provide a "Debug mode" setting to choose which set is active.