cart tweaks

- Fixed './cart.php' UI when viewing the page in FireFox
- added method CheckoutManager::getOrderTotal; it returns an associative
array with the keys: 'sub', 'tax', 'ship', 'grand'
- './cart.php' now utilizes CheckoutManager::getOrderTotal
ui tweak

- cart items table is no longer striped; it looked weird with the white
background of the image
Merge pull request #99 from uptownjimmy/cartStyles

Cart styles