make ship_accept responses link to packages

The ::Response::PackageResult objects in a ::Response::ShipmentAccept
now link back to the ::Package objects they relate to. This is based on
the wide-spread assumption that the result elements are in the same
order as the request elements. Everybody seems to rely on this, even if
I can't find an official confirmation.
kill spurious Data::Printer in code
add linear_unit and weight_unit to ::Package

measurement_system is now a shortcut in the constructor parameters. Note
that using kilograms/inches or pounds/centimetres will currently result
in an error message from UPS.
use Test::NoTabs
no need to document BUILDARGS separately
make ::Package->id a string
v1.0.5

 - the ::Response::PackageResult objects in a
   ::Response::ShipmentAccept now link back to the ::Package objects
   they relate to
 - ::Package now has separate linear_unit and weight_unit attributes;
   measurement_system is now a shortcut in the constructor
   parameters. Note that using kilograms/inches or pounds/centimetres
   will currently result in an error message from UPS
 - the 'id' attribute of ::Package is now a string; it's no longer
   used internally, so it's now a generic user-side identifier