fix reverse charge support (eu)
The reverse charge support in blesta is broken/wrong!
Currently invoices for vat exempt businesses show 0% vat,
where it should not show any VAT!
this means that we are incorrectly charging 0% tax vs not charging it.
(invoice needs to show a note of "reverse charge rules apply" in this case, wich can be done currently via the extra nots/comments/text in invoice cumtomisation).