e621dl 5.9.0

New API adaptation and login support

• So, e621.net moved to a new engine. That means
  more tags at a time. Also, it means pain to remake
  all relevant parts. Luckily, posts have internal
  representation that could be mapped to the new API
  leaving most of the logic intact.

• Since some posts are now hidden for the guest users,
  I added simple login and api_key settings to the
  settings section.