Closes #93

- reworked the code in file  './cart.php'
- the cart page now displays the subtotal, sales tax, shipping cost, and
grand total of all items in the cart
Small Tweaks #93

- './cart.php' only displays the 'checkout' button if there are items in
the cart
- './cart.php' now redirects the user to the appropriate page depending
on if the user is currently logged into an account
More Tweaks #93

- './guestcheckout.php' now accepts the order total as POST data under
the key 'data'
- './guestcheckout.php' now stores POST data under key 'data' into a
hidden field within the 'guestUserForm' HTML form
Merge pull request #96 from uptownjimmy/#93

#93