Issue71 contact key serialization

Closes #71.
The contact has still to be added to the de-/serialization test of the complete Settings. I will add this in another PR as there has to be implemented a TypeAdapter for SyncedSettings.
Why is it possible to set the owners key id w/o setting the owner itself? I guess this is the same thing mentioned by @cburkert. Is this needed for (de-)serializing?