currently tax is calculated before promotion, added am option in the productstore to be able to set the tax calculation after promotion. This in order to remain backward compatible. Also added a junit test for this. Contribution from Tom, employee from Antwebsystems

git-svn-id: https://svn.apache.org/repos/asf/ofbiz/trunk@1449615 13f79535-47bb-0310-9956-ffa450edef68