TimestampColumnDateTimeMapper uses null to calculate adjustment

fixes #7 

In TimestampColumnDateTimeMapper#toNonNullValue the following adjustment calculation can be found:

        int adjustment = TimeZone.getDefault().getOffset(value.getMillis()) - currentDatabaseZone.getOffset(null);

I would expect the adjustment to be 0 if TimeZone.getDefault() and currentDatabaseZone are "Europe/Berlin". But:

        DateTimeFormatter dateStringFormat = DateTimeFormat.forPattern("yyyy-MM-dd HH:mm");
        DateTime value = dateStringFormat.parseDateTime("2013-09-17 16:00");
        System.out.println("value.zone: " + value.getZone());

        DateTimeZone currentDatabaseZone = ZoneHelper.getDefault();
        System.out.println("db.zone: " + currentDatabaseZone);

        TimeZone timeZone = TimeZone.getDefault();
        System.out.println("java.zone: " + timeZone.getID() + " dstSavings: " + timeZone.getDSTSavings());

        int adjustment = timeZone.getOffset(value.getMillis()) - currentDatabaseZone.getOffset(null);

        System.out.println("java.zone.offset: " + timeZone.getOffset(value.getMillis()));
        System.out.println("db.zone.offset.null: " + currentDatabaseZone.getOffset(null));
        System.out.println("db.zone.offset.value: " + currentDatabaseZone.getOffset(value.getMillis()));
        System.out.println("adjustment: " + adjustment);

prints:

    value.zone: Europe/Berlin
    db.zone: Europe/Berlin
    java.zone: Europe/Berlin dstSavings: 3600000
    java.zone.offset: 7200000
    db.zone.offset.null: 3600000
    db.zone.offset.value: 7200000
    adjustment: 3600000

If I change the initial date to November, the adjustment is 0 (as expected)

    DateTime value = dateStringFormat.parseDateTime("2013-11-17 16:00");

I think the problem is caused by the initial date being before the daylight savings time switch and today is after the daylight savings time switch. If I replace `null`with `value.getMillis()` I always get a 0 Milliseconds adjustment:

     int adjustment = timeZone.getOffset(value.getMillis()) - currentDatabaseZone.getOffset(value.getMillis());

Or am I missing something?