In class SyncedSettings:
* added field contacts
* added setter and getter for field contacts
* added initialization of contacts to class constructor
In class Identities:
* changed collection type from Set to Map in order to enable retrieving identity by key identifier
* adapted change of collection type in add() method
* added getter getIdentityByKeyIdentifier()
new class SyncedSettingsTypeAdapter
In class ConfigSerializationTest:
* split tests for SyncedSettings and LocalSettings
* changed test for SyncedSettings to use new SyncedSettingsTypeAdapter
in class Identity:
* added annotation SerializedName("keys") to field primaryKeyPair