*Title: Allowing objects to control VAT information

*Incentive:
Make it possible to change VAT selections at object level.

*Documentation:
Up until eZ Publish v. 3.5, it has only been possible to make selections with
regards to VAT at class level. This modification allows the user to specify
VAT type and whether the specific price is ex or inc VAT at object level,
allowing products with different VAT types to be a part of the same class.

The VAT selections made at class level will be the default selections when
creating objects based on an attribute of the ezprice datatype.