complete Orders section of API metadata
complete Orders section of API metadata

- add ListOrdersByNextToken, ListOrderItems, ListOrderItemsByNextToken,
  GetServiceStatus
- add throttle info to endpoint data
- add return info to endpoint data
- none of this is currently used, but since i'm digging through the
  documentation, might as well add it now for future use

ericblade/mws-advanced,ericblade/mws-advanced