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 
Why?
From default variable success is set as true. So if you do not nothing success will return true. The variable checked is used that I know that this function is called min. one time

In public boolean isSuccess() you will see it

If you want I will change it.