Fixed issue with incorrect datetime-epoch conversion. Existing time.mktime() assumes the provided datetime tuple is local. Replaced by calendar.timegm(), which assumes UTC datetime.
Fixed issue with incorrect datetime-epoch conversion. Existing time.mktime() assumes the provided datetime tuple is local. Replaced by calendar.timegm(), which assumes UTC datetime.

hozn/stravalib,gcmartinelli/stravalib,matt-leach/stravalib,Wisees/stravalib