Multi-Currency Support for payment gateways

Multi-currency support is becoming a fairly popular request for our gateways. Let's investigate how we can implement a nice abstracted approach to multi-currency.
## Gateways that support multi-currency
- **Authorize.net (AIM/CIM)** - requires a separate merchant account (API login ID/transaction key) per currency. API endpoint does not change. See [WooIdea](http://ideas.woothemes.com/forums/133476-woocommerce/suggestions/5168165-authorize-net-support-for-multiple-currencies)
- **Beanstream** - requires a separate merchant account (ID) per currency. API endpoint does not change.
- **Braintree** - requires a separate merchant account (ID) per currency. API endpoint does not change.
- **CyberSource** - no special configuration required, include currency in transaction request.
- **Elavon Converge** - no special configuration required, include currency in transaction request.
- **First Data/Payeezy** - no special configuration required, include currency in transaction request.
- **Moneris** - requires a separate merchant account (store ID/API token) per currency. API endpoint does not change.
- **PayPal Express** - no special configuration required, include currency in transaction request. [Details here](https://developer.paypal.com/docs/classic/api/currency_codes/)
- **Realex** - no special configuration required, include currency in transaction request.
- **USA ePay** - requires a separate merchant account (source key) per currency. API endpoint does not change.
## Summary

5 gateways that require separate credentials per currency. None that change API endpoints. Some of those accept a lot of currencies (Braintree [supports over 100+](https://developers.braintreepayments.com/ios+ruby/reference/general/currencies)), so I think our implementation would have to assume the currently entered credentials are for the store's base currency, and allow the admin to dynamically add additional credential fields for a selected currency. These should also be keyed to the environment field.

The framework doesn't have any concept of a credential field so this would have to be abstracted into perhaps a method that returns an array of those credential fields given a specific currency (either defined or from an order) so implementing gateways can parse them into whatever credentials it needs.