Use the new PriceList class

Signed-off-by: Tomas Hozza <thozza@gmail.com>
Sort prices list if is is set using constructor

Signed-off-by: Tomas Hozza <thozza@gmail.com>
Replace tests for Price class with tests for PriceList class

Signed-off-by: Tomas Hozza <thozza@gmail.com>