WIP: porting Spree's API to Active Model Serializers

Beginning with the products controller
Convert variants controller to use AMS
Convert variants controller to use AMS
Convert address controller to use AMS
Convert CheckoutsController to AMS
Port config controller to AMS
Port countries controller to AMS
Port over images to AMS
Port inventory units to AMS
Port line items to AMS
Port option types to AMS
Port over option values to AMS
port orders API to AMS
Port over payments API to AMS
Port product properties API to AMS
port properties API to AMS
Port return authorizations API to ams
Port shipments API to AMS
port states API to ams
Port stock items API to AMS