Limit air-by-date searches to in-season eps.

If searching for air-by-date episodes, limit the DB search to Seasons only (not Specials). Occasionally air-by-date's have a special same day as an ep. This breaks the search.

eg. [Last Week Tonight with John Oliver](http://thetvdb.com/?tab=series&id=278518) Episode 1, Date 2014-04-27 conflicts with a Special on the same day.