Add to cart and minimal prices

1. Need to make sure that for multisku the default sku selected corresponds to sku with minimal price (so that add to cart add the sku with displayed price)
2. Need to change the AbstractSkuCartCommandImpl.setProductSkuPrice to find minimal price for CartItem skuCode NOT! minimal sku from that product
3. CartItem must have additional getListPrice() method that will return the original price without discount, so that we can show how much people actually saved, this method also needs to be added to ShoppingCart class so that we are able to show full savings on the cart
4. Need to check the bug for adding multi tier sku YC-170