Recommend using billing address as basis for determining customer country.
Refactored FixedDiscount tests in WebServiceRowFormatterTest, removing redundant test cases and adding more test cases.
Refactored formatFixedDiscountRows, broke out formatFixedDiscountSpecifiedAsAmountIncVatOnly to its own method.
Added formatFixedDiscountSpecifiedAsAmountExVatOnly()
Updated documentation in class FixedDiscount, and createOrder->addDiscount() usage in README.md.
Refactored WebServiceRowFormatter, updated unit tests.
Merge branch 'feature/FixedDiscount_specified_ExVat_INTG-312' into develop