added ShipmentServiceLevelCategory to Order
Corrected Query to deal with multiple markets

According to the Amazon MWS specification, multiple marketplace ids are
listed as MarketplaceId.Id.1,MarketplaceId.Id.2,etc. and not
MarketplaceIdList.Id.1, MarketplaceIdList.Id.2, etc.
Merge branch 'master' into add_order_ack

Conflicts:
	lib/mws/query.rb
added SellerOrderId