support for merchant_order_id

2checkout has two parameters that are used for specifying order id.

One is order_cart_id(the one that omnipay uses). However, it's not sent later by the Instant Notification System (INS).
The other one is `merchant_order_id` and it's sent by the INS(with a different name - `vendor_order_id`). 
I think it's ok for both parameters to share the same value.