update the calender.js event loader routine

I'm not fixing any bugs with this, but it seems more reliable/logical to me to do this.

assuming that:
 a) an episode's TVDB_ID is unique, and 
 b) trakt.tv would never provide an episode without its TVDB_ID,
 then given the calender events are episode elements that should never have duplicates,
 it makes sense to use its TVDB_ID instead of (episode.serie_ID && episode.firstAired) during calender event loading.
If assumptions (a) and (b) are incorrect (or you prefer the current code) then do not accept this patch :-)