invoice submitting not possible #6

Hello. Thanks for all the work on this library, you saved me a bunch of trouble, and came up with a pretty well designed solution, where I would've done a quick and dirty fix.

I notice none of the PayflowProObject subclasses provide an invnum parameter that paypal accepts for providing the store's internal invoice for the sell.

I don't think it's a required parameter for any transaction, but I thought maybe it'd be a nice thing to have. Do you have a reason for not adding it?