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 
Since this is the state of the whole shipping I guess the mentioned thing is really wanted here. Am I right? But this does not explain my question [above](https://github.com/Qabel/qabel-core/pull/217/files#r21181131)