subclasses dynamically (Calculator::Fedex::Base.descendants, etc.)
locale file since it is no longer being used in the compute method (we're using rates[self.class.description] instead now) and to be more consistent with how we're doing it for UPS and FedEx. Added calculators for PriorityMailInternationalSmallFlatRateBox, PriorityMailInternationalMediumFlatRateBox, and PriorityMailInternationalLargeFlatRateBox. Renamed PriorityMailRegularMediumFlatRateBoxes to PriorityMailMediumFlatRateBox to be consistent.
services from "Flat-Rate" to "Flat Rate" to match the key used by USPS's response, so that rates[self.class.description] will successfully look up the rate instead of returning nil.