strptime bug fix: if gmtoff is None we have 1 hour increment of time

(through utctimetuple), compare:

>>>> datetime.datetime.fromtimestamp(time.mktime(datetime.datetime.now().timetuple())).strftime("%Y-%m-%d %H:%M:%S")
'2014-04-29 17:26:31'
>>>> datetime.datetime.fromtimestamp(time.mktime(datetime.datetime.now().utctimetuple())).strftime("%Y-%m-%d %H:%M:%S")
'2014-04-29 18:26:37'