Update JSONCallback.java

This change supports datetimes in different timezone.
You can insert for example this document:
		String o = "{"
				+ "\"fdata\" : {$date: \"2011-11-21T14:30:00.123-0400\"},"
				+ "\"fdat2\" : {$date: \"2011-11-21T14:30:00.123Z\"},"
				+ "\"fdat3\" : {$date: \"2011-11-21T14:30:00-0400\"},"
				+ "\"fdat4\" : {$date: \"2011-11-21T14:30:00Z\"}"
				+ "}";
	DBObject dbo = (DBObject)JSON.parse(o.toString());
        coll.insert(dbo);
and obtain four dates.