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.

delete "the". whitespace around the = sign. Probably mention that the protocol can be extended in various ways, this is only one.