Cart: Naming cleanup - removed total as expression

As this is given implicitly anyway for all direct properties on the cart
itself
Cart: More naming changes and cleanup

Restructured cart constructor for clarity and consistency.

New naming standard:
* `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

Also prettified toString methods
Merge pull request #75 from iZettle/cart-naming-improvements

Cart naming improvements