Allow for addition of promo/discount codes to reduce registration costs

In the mobile optimised, payment up front registration process, it would be good to allow the user to enter a promo code which could reduce the cost of the registration.  These promo codes should be applicable to either all leagues or just a subset, should apply either a percentage discount or a fixed amount off, and should be displayed as a discount on the statement.  It should also be possible to allow the codes to be applied only by certain people, or for certain teams, and it should be possible to define the date range during which the code is valid.