#548594: VAT-ID is optional, not a must, patch http://drupal.org/node/548594#comment-3911856 by antiorario

- cache result of succesful validations for one day
- use the SOAPClient library (although only the nuSOAP client seems to work, 'SOAP Extension on PHP5' and the EU VAT validation service don't seem to want to work together)
- autocomplete on VAT field (taken from the current user's previous orders)
- added VAT field to admin order interface
- set Ubercart conditions, which can be used by Tax rules for conditional VAT application (using ICV / intra-community rules)
- VAT number and company are never required
- VAT number is in its own fieldset, so the JS showing/hiding of the billing address doesn't pose problems anymore
- And of course the leading 0 was removed, I also had the JS version of the countries array generated from the PHP one so there's no duplication