Change OrderProducts to OrderItems in Order.cs

[mikelarkin]

1. The attribute was incorrectly renamed to OrderProducts. This needs to be reverted to OrderItems.

2. As per the documentation, the API no longer returns an array of order_items, but rather a URI that needs to be called to retrieve them, order_items_uri. The OrderItems method therefore needs to invoke the order_items_uri and return the results as an array.

3. Add order_items_uri as an attribute

The idea is that a user should still be able to call myOrder.OrderItems and get an array of OrderItems back.