Balanced Payments store method doesn't return account_uri

The original implementation of the store method for Balanced only returned the card_uri of the created/stored card. I'm proposing that this be changed to a Hash so that the account_uri can be passed back as well (it's either looked up or created). This is necessary in cases when a new account is created and the caller needs to store that.

My only concern is that in some circumstances, a CreditCard was being returned (I think in the case where it was passed in as a parameter to store). I'm not all clear on what that path was intending. Maybe someone can shed some light?