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.
According to the [documentation](https://github.com/Qabel/qabel-doc/wiki/Components-Crypto#key-identifier) the key id has to be the *right-most* 64 bit of the fingerprint. Your solution seems to take the left-most.