cleanup billinginfo.cs, verified endpoints match api spec
cleanup up coupon
cleanup couponredemption
clean up errors and exceptions
cleanup invoice
clean up of Plan
clean up AddOn
clean up Subscription; fixed typo in SubscriptionState, made SubscriptionState a Flag enum to support multiple states
clean up of Transaction
added centralized enum parsing to compensate for service/client mismatch in naming conventions, replaced occurrences of raw parsing to use new style