Complete subscription support

[bastienho]

This PR make other payments in a subscription reliable.

Explanations:

• When vads_trans_id is sent in the request, it's also return its value in the IPN
• But in a subscription context, on others payments, the vads_trans_id is replaced by a generated transaction ID

My approach is to populate the vads_order_id field with the same value as vads_trans_id and use it as internal transaction id. The value of this field is kept over all future payments.

In the meantime, we add a getTransactionReference() with the value of vads_subscription_id which allows to attribute new transaction to the correct subscription.

@damsfx does it make sens?

I have a doubt about the duplicate value in order_id and trans_id.

[damsfx]

@bastienho Sorry for the late reply, I've got a very busy schedule.

Have you been able to test it in a real-life situation? I don't have any cases where I can test your modifications.

I have a doubt about the duplicate value in order_id and trans_id.

Mee too.

Perhaps you can add the required tests to your PR?