Add organization_ids property

[NecatiMeral]

Fixes #47

zammad offers the organization_ids property which contains the IDs of the secondary organizations.

{
    "id": 1463,
    "organization_id": 1058 /* Test-Organization-One */,
    "login": "nm@lis-gmbh.com",
    "firstname": "Necati",
    "lastname": "Meral",
    "email": "nm@lis-gmbh.com",
    // ...
    "organization_ids": [
        1059 /* Test-Organization-Two */,
        1060 /* Test-Organization-Three */
    ],
    "authorization_ids": [],
    "karma_user_ids": [],
    "group_ids": {}
}