Using calendar.timegm instead of time.mktime.

Note that time.mktime says:
"Convert a time tuple in local time to seconds since the Epoch."
while calendar.timegm says:
"handy function to calculate Unix timestamp from GMT."

This is why tests were failing with local timezones seeming
to matter (because they did with time.mktime).

Also changed import of datetime in datastore._helpers
to make namespace/class confusion clear. I noticed this when
looking for other uses of calendar.timegm in the codebase.