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).

Do you understand this?


Off-topic:
I know the problem with the style guide -- please have a look at #40 and help with it.
One pro-tip: No one cares who created a class because it is totally irrelevant and no one should know who created a class by reading its **CODE!** **Never**. You really should not need a guide in this repository to tell you this basic principle of software engineering. Your argument "This is my class" is the **worst** thing you could reply to this.