Add self.get_updates(callback, period).

I needed both series and episode updates, so I wrote a new method that returns all update info at once: get_updates().  It avoids a bug in get_updated_shows() (which returns a bunch of None elements from the nested Series data in Episode and Banner records).  It also makes the update time available (via the "time" attr).  Finally, it changes the URL used for all get_update*() functions to fetch the zip-file version, for reduced transfer sizes.  As for speed, when I processed a week's worth of updates, the runtime for my script went down from 1 second to about 0.5 seconds (compared to a non-expat functiom that returned both episode and series data via the ET method the other functions use).