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 

> You did not answer my question or I did not understand your answer
> thoroughly. My question is whether it is correct that this method will
> returns true EVERY time since it has returned true ONCE not why you
> used a boolean and that the other side doesn't need to know the error
> code (I totally understand that).
It is correct! And I update the comment in the return value. Is that
answer enough?

Next time I answer on any comment.

> Do you understand this?
Yes.