Add Video#video_cookies, and cookie_jar{,_file}

- Marking get_video_by_other deprecated
v1.5.0

= Enhancements

- Added Niconico#logged_in? for natural English version of `logined`
  method
- You can now re-login again automatically only when necessary.

      nico = Niconico.new(email: 'xxx@xxx', password: 'secret',
                          token: 'user_session_***')

      nico.login

      p nico.token # Keep given `token` basically, but acquire by
                   # re-logging in automatically, when it's necessary

= New method

- Expose current user_session in method Niconico#token.

= Deprecations

- Video#get_video_by_other has marked deprecated.

  Use Video#video_cookies, and Video#video_url instead.

  If you're using curl and curl cookie jar for video downloading,
  Video#video_cookie_jar or/and Video#video_cookie_jar_file method may be
  helpful.