Discount per item

Consider this as a suggestion for how we should handle item specific discounts. Appreciate your thoughts @mikaellothman and @softarn !


So, although we'll never have decimal discounts as you described, I still think I agree with you in principle :)
Looking at the rest of the cart code, we're always calculating and rounding the discount, then subtracting it from the gross amount. For example in the cart constructor itself. 

We should definitely do it in the same way when it comes to local discount calculations!

BRB (have to rebase this anyway) :)