Merge pull request #91 from mrpollo/1-3-stable

removed raise ShippingError from compute, fix #90
usps api name changes fix #103
moving from matching service names (desc) to checking for service codes
in order to achieve this it was necessary to override compute and
retrieve rates from Spree::Calculator::Shipping::ActiveShipping::Base on
Spree::Calculator::Shipping::ActiveShipping::Usps::Base
I added a service code paramter to all the usps classes and made sure
it was the correct id, please refer to this issue for further
explanation: https://github.com/spree/spree_active_shipping/issues/103