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 as well as the time attr
attached to the Data element.  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 ET).