Continue Shopping Closes #35

- './cart.php' now has two 'Continue Shopping' submit buttons; both
buttons redirect the user to './index.php'
Checkout Validation Closes #37

- './details.php' now checks if an account is logged in before writing
'Add to cart' button data to the page; 'Login' and 'New User' buttons
are now written to the page if an account is not logged in
Empty Cart Checkout Closes #36

- './cart.php' now checks if the cart is empty before writing the page;
if the cart is empty, './cart.php' will no longer write the total of the
cart or the checkout submit button to the page
Revert "Empty Cart Checkout Closes #36"

This reverts commit 66e3f80ce3f907d590721a4a799783f79d1bc7f9.
Empty Cart Checkout Closes #36

- './cart.php' now checks if the cart is empty before writing the page;
if the cart is empty, './cart.php' will no longer write the total of the
cart or the checkout submit button to the page
- added method CartManager::isEmpty; returns TRUE if there are no items
in the cart
Merge pull request #40 from uptownjimmy/cartEmptyCheckout

Cart empty checkout