fix tax calculation for prices entered inclusive tax 

see
http://easydigitaldownloads.com/support/topic/tax-issue-still-exists-if-prices-are-entered-including-taxes/
and
http://easydigitaldownloads.com/support/topic/tax-calculation-wrong-for-inclusive-taxes-patch-included/
Merge pull request #1460 from robaggior/patch-6

fix tax calculation for prices entered inclusive tax