Add a third optional parameter to the weight and dimension normalization
functions in order to be able to specify a specific "from unit" instead
of using the site option. This allows arbitrary conversions between
different units of measure; which is particularly useful for extensions
that need to perform arbitrary unit conversions (e.g. shipping extension
that specifies custom box formats).
Also includes updated tests.
Mike Jolley
Joey Kudish
Claudio Sanches
Nicola Mustone
Barry Kooij