Couchdb#put! -> Couchdb#record_job.  #18.

ArchiveBot's CouchDB database will now be storing archive URLs, so we
can't assume that a PUT means "save this job".  (Technically, it could
have also meant "save ignore pattern set", but we've ignored that.
Until now.)
Introduce ArchiveUrl.  #18.

Main motivation: document an ArchiveUrl's fields.
Introduce ArchiveFinder cog and IA scraper.  #18.

Doesn't yet save any archive URL records, though.