fix broken clean_transmission plugin for seeding only torrent see #2803

by looking at the date_added time of the torrent. 
Date added should usually be less than date completed, but if one adds a torrent only for seeding (so all files are completed before adding the torrent) the date_finisehd field is not written.

This PR could be altered by someone more experienced to also remove seeding torrents after given timeframe.
Currently seeding only torrents are not deleted.
This is clause is starting to look like a monstrous mess. Care to cleanup it a bit? Each of those rules should be assigned to a descriptive variable. Imaginary example:

    is_expired = (torrent.date_added + nfor) <= datetime.now())

then the if clause will read nicely

  if is_expired and is_foobar or is_crap