Let handles worry about how to represent themselves in SOAP data
Allow Handle to model currency and VAT handles

Basically this means Entity::Handle now supports :code and :vat_code keys
Handle properties accepts Hash inputs

Even though we're moving towards handles being full-fledged objects, we'd still
like to preserve the existing, easier way of setting handles as Hashes.

With this commit the following two are equivalent:

  Debtor.new.currency_handle = {:code => "USD"}
  Debtor.new.currency_handle = Entity::Handle.new({:code => "USD"})