[gpupdate.rb] refactoring. use IMAGE_STATE_* instead of integer literal(0..6)
[gpupdate.rb, lib/demobels.rb] ignore images anyway if image_type = 'youtube'
[gpupdate.rb, lib/dbmodels.rb] change rules about download-failed images

  - first, set status to IMAGE_STATE_FAILED
  - ( we will retry to downlaod every day. )
  - 3 months after the post was published, set status to IMAGE_INVALID_URL
  - then no longer try to download...
[gpupdate.rb] add comment for command-line options