Add "atleast = 1" arg to get_show_and_episodes.

In my use of get_show_and_episodes(), I find I only need newer episodes
to be returned (it is focusing on future episodes), so I added an arg to
get_show_and_episodes() that allows an episode ID to be specified (it
defaults to 1).  This value will stop the routine from returning any
episodes_id < atleast.
Added self.last_updated_utime to Episode and Show.

For tasks such as comparing update times against the last_update value,
we need the raw epoch-seconds value for last_updated.  In order to avoid
needing to try to convert the datetime value back into unix time (and
possibly getting a localtime epoch instead), I stashed the raw int value
in self.last_updated_utime (leaving self.last_updated as it was).