Addition to gem to allow account overriding

Hi Paul,

I've made another addition to the RealEx gem - this time to allow account overriding.  Why would this be useful?  Well as we've just found out if you want to accept American Express is has to be under a different subaccount than the normal settling bank.  Therefore you need to switch accounts on a per request basis depending on whether the customer has a visa/mastercard or a amex card stored.

The change to the gem is very small thanks to the way it was originally created.  Comes with appropriate spec.

George