Event 15 - Payment amount cannot be null 

@ryancarlson last of my current ERT Jira tasks http://jira.uscm.org/browse/EVENT-15
Actually, what is somewhat redundant is the the new `validatePayment(...)` method, since there already existed the 'validatePaymentReadiness(...)`.  However, I needed some validation to apply not only when processing credit card payment, but also when `saveNewPayment(...)` as well as on credit card refund. I wasn't thrilled with this solution, but it was the best I could think of.