Add Pop method to ZmqMessage

[mrijkeboer]

I've added a Pop method to ZmqMessage that is equivalent to zmsg_pop(). We need this method since Unwrap also deletes the second frame when it's empty and this is sometimes unwanted e.g. when the payload frame after the header is null but we still want to submit the empty payload frame to the higher layers.

[TimGunnells]

Thank you. I needed this.