Refactor Order factory pattern to be consistent with Payment and Shipping classes

[sharvey]

Order still uses a factory concept of Order::create() which is redundant, and makes no sense when the Payment and ShippingCalculator? classes use a different system.

Object::create('Order') should be used instead, like the aforementioned classes, which means we can use Object::useCustomClass() in our _config.php files. This means a net loss of code in Order, too.