VAT number request on user registration

Issue Summary

Hi there,

i thought about this a long time. I think if business users buy several times a month in the same store and if they have to fill in their VAT number each time - they will stop buying in this store ;)

So maybe under "admin/store/settings/store/edit/contact" there should be two options (instead of one at the moment):
- Check the validity on the customer VAT Number on checkout via Europa VAT Number validation webservice
- Check the validity on the customer VAT Number on user registration via Europa VAT Number validation webservice

What do you think?