Support custom fields

[judgej]

This is a collection of name/value pairs. Possibly a set/get method to handle the whole array. Some other way to handle individual fields to add to the collection would be good.

Conceptually, a magic getter/setter would catch any fields not caught by the core parameters, and get/set them as custom parameters. That may fight some of Omnpay's approaches, but it's the way the gateway expects to work.