justin: support archived content from twitch and justin

Archive contents have URLs that look like justin.tv/<channel>/b/<id> and
twitch.tv/<channel>/b/<id>. Otherwise, the implementation is equivalent with
that of _get_video, which is now renamed to _get_chapter.

Closes #67.
justin: adjust error msg in case of type being unknown
fetcher.http: send url to debug log before request
Merge pull request #68 from olof/topic/justin_archive_content

Support archived content from Justin/Twitch