Add Insured Value field to the package line items definition

We just faced a troubles passing FedEx certification for the shipment service API because of empty Carriage Value field, so, here it is :)

This only present on the international labels.

    packages << {
      :weight => { :units => "LB", :value => 2.425082 },
      :insured_value => { :currency => 'UKL', :amount => 100 }
    }

![label pdf__ _1__3_](https://f.cloud.github.com/assets/62760/1225365/ebaa1bd8-2764-11e3-90d0-e1dc719640be.png)