Implement 304-handling
This passes the time of the last feed refresh to feedparser.parse. If the feed has not changed since the last refresh, most servers will answer with 304: Not Modified. In this case, feedparser does not parse the feed and aborts, which speeds up refreshing the whole library.
time.mktime() works with localtime whereas feedparser expects UTC. Hence, I replaced time.mktime() with calendar.timegm().