Optional info field in UpdatePayment message

To simplify multiplexing payment update over one payment channel, an optional ByteSting info field is included in the UpdatePayment message. If a client chooses to include some info in the increasePayment call it will be delivered in the paymentIncrease callback of the ServerConnectionEventHandler interface.

Well, thats a matter of programming style. I think I've been doing to much Scala... But I'll adopt.