Support reference numbers on package and shipment.

For whatever reason, the UPS API does not support package level reference numbers for shipments within Canada.  However, they do support reference numbers at the shipment level.  This change gives you the ability to specify (as an option) the reference number.  However, you must nest this within a :package or :shipment hash.

ups = ActiveMerchant::Shipping::UPS.new(config)
ups.obtain_shipping_labels(from, to, boxes, {
  # other options...
  :package => {
    :reference_number => { :value => order.id, :code  => "PO" }
  }
})