Added self.last_updated_utime to Episode and Show.

For tasks such as comparing update times against the last_update value,
it's easiest to use a 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).