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 
> It is correct! And I update the comment in the return value. Is that answer enough?

Yes -- thanks for the answer.
Some Javadoc about this could help to avoid confusion.

> Next time I answer on any comment.

Well yeah. That's the idea ;)