Closes #182

- CheckoutManager::getOrderTotal now checks the shipping method when
determining the shipping cost for the order
- './cart.php' and './checkout.php' now display the shipping cost as
'Estimated Shipping'
fixing notice

- now checks to see if the session key 'summary' exists before
calculating shipping costs
Merge pull request #188 from uptownjimmy/shippingOptions

Shipping options