Cart naming improvements

* `total` is removed as this is given implicitly anyway for all direct properties on the cart itself.
* `amount` is removed and to some extent replaced by `value` where appropriate.
* `effective` is replaced by `actual` as it's deemed clearer according to PR feedback

So, running the same example as in the discoussion in PR #74  and printing the cart.toString() value gives:
```
Cart {
	LineItems:
		LineItem { item = TestItem{ unitPrice = 9000, vatPercentage = 25.0, quantity = 1.0, name = Car}, grossValue = 9000, grossVat = 1800, actualValue = 8100, actualVat = 1620}
		LineItem { item = TestItem{ unitPrice = 1000, vatPercentage = 12.0, quantity = 1.0, name = Apple}, grossValue = 1000, grossVat = 107, actualValue = 900, actualVat = 96}
	Discounts:
		DiscountLine { discount = TestDiscount{ amount = null, percentage = 10.0, quantity = 1}, actualPercentage = 10.0, value = 1000}
	Gross Amounts:
		Gross Value: 10000
		Gross VAT: 1907
	Actual Amounts:
		Value: 9000
		Discount Value: 1000
		Discount Percentage: 10.0
		VAT: 1716
}
```
ping @MrShish or @xiaodong-izettle again, and @idlefingers for inclusion. Sorry to bother you again:)