Added functionality to create subscription requests

Allowed amount = 0 in case a subscription is initiated (see PaymentRequest.php and simpleForm.php)

UPDATE 31-07-2012: Reply from Ogone about amount 0 for initial payments. This is something Ogone can enable and they have done so for our test account. If this enabled you can create payments with amount 0. This update I've provided works like a charm now that they've enabled it. Perhaps something that can be added to the documentation as well?

UPDATE 30-07-2012:
Awaiting feedback from Ogone. Their documentation doesn't seem correct:

"Tip: If the merchant does not wish to perform an actual transaction at the time of the subscription
creation, he may submit an amount of 0 €, which will then only validate the card;the transaction will
not be submitted to the acquirer."

However, this results in an error when you set the initial payment to 0.