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()
* added javadoc for new and modified methods
new class SyncedSettingsTypeAdapter
In class ConfigSerializationTest:
* split tests for SyncedSettings and LocalSettings
* changed test for SyncedSettings to use new SyncedSettingsTypeAdapter
* changed URLs in contactTest to match the DropURL format
in class Identity:
* added annotation SerializedName("keys") to field primaryKeyPair