fix bug with date strings without timezone

Example:
    document.write(new timezoneJS.Date('05/29/2013 09:25', 'Europe/London'))
Expected output independent of client's timezone:
    2013-05-29 09:25:00
Output without this fix:
    [date/time in timezone `tz`, where `dt` is interpreted in client's timezone]

Note: without this fix, it is not possible to get a unix timestamp, if
the only available information is a datestring and a timezone string
(e.g. 'Europe/London').