Refined lastModified timestamp attribute in JSON output

It should be rendered like the "registered" attribute, e.g. as

lastModified": {
    "millis": 1362413421741,
    "value": "2013-03-04T16:10:21Z"
}

Change-Id: I2a55f43a2a44b5cc14bc19f45264dee9a9f19f79
Signed-off-by: Michael Ochmann <michael.ochmann@sap.com>