FIX: include all categories in category discount constraint filtering
Refactored discount calculator.
Removed option to link discounts or not.
Introduced a way to specify the 'discountable amount' on an order item (does not include quantity).
Added logging to calculator to help with debugging.
Refactored tests to be more readable, and probably faster.
Failing test "testItemAndCartLevelAmountDiscounts" is known it uncovers issue #21
Remove empty test file
Include shop shipping module in dev requirements. This is so it can be included in travis unit tests.
Rearranged test fixtures and tests in accordance with shop changes.
Updated CategoriesDiscountConstraintTests to also test orders that contain variations
FIX: don't let item+cart discounts go beyond the subtotal of the order.
fixes #21