Fix #111 Drop::sendAndForget can return a false value
Move DropResult and DropResultPair to a new class
Update DropMessage<T> to DropMessage<? extends ModelObject>
in class SyncedSettings: * added new field contacts * added getter and setter for field contacts * added new field initialization in 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()
in Class ContactsTypeAdapter: * use now proper ContactTypeAdapter in write() and read() methods
new class SyncedSettingsTypeAdapter
in class Identity: * added annotation SerializedName("keys") to field primaryKeyPair
Merge pull request #159 from k-c13/drop-send

Fixed #111 - Drop.sendAndForget can return a false value