Fixed #111: Drop::sendAndForget can return a false value

Add new classes DropResultContact and DropResult 
Fixed #111: Drop::sendAndForget can return a false value

Close #195 
This return true. And this is excatly what I want. So the user of this function can choise whether it want to stop sending messages. The other side does not need to know the error code. In a derived class it can handle other error codes which are also correct in other cases. Maybe it is a little to scary for other.