SessionSingleton Class Implementation

- Moved folder './Cart' to folder './php/cart';
- Created folder './php/cart/ajax'; stores PHP scripts that should be
called via AJAX
- Moved file 'cart_add_item.php' to folder './php/cart/ajax'
- Moved file 'cart_remove_item.php' to folder './php/cart/ajax'
- './php/cart/ajax/cart_add_item.php' now uses relative file path to
load its own dependancies
- './php/cart/ajax/cart_remove_item.php' now uses relative file path to
load its own dependancies
- './php/cart/CartManager.php' now uses relative file path to load its
own dependancies
- './header.php' now uses relative file path to load its own
dependancies
- Created './php/SessionSingleton.php'; abstract class which acts as a
singleton, but is persistent across multiple script calls via session
- './php/cart/CartManager.php' now extends './php/SessionSingleton.php'
- Provided better comments for QA