Support multiple merchant accounts

[arjun289]

The Authorize.Net gateway only supports USD for now.
Create the functionality to support other currencies.

[oyeb]

Do you mean to support multiple ANet accounts or has this been resolved because of #62?

[oyeb]

AFAIK, Authorize.Net allows a merchant to use just one currency for all operations. To work with multiple currencies, one must make several accounts with them. See this.

Gringotts can very easily support multiple merchant accounts, even with the same gateway. The user should explicitly pass in their credentials under the :config key in opts -- overriding any config found in the application-config.