Retrieve archive URLs with job history.  #18.

This commit also drastically simplifies Couchdb#history's interface,
which is both good and bad.

Good: simpler invocation.  Bad: we lose the ability to do things like
limiting result sets, which is actually driven by a limitation of the
view used by #history.

(This, by the way, is probably a good example of where you really want a
traditional relational database.  However, IMO, the cost of conversion
isn't yet worth it.  Enjoy this rather stupidly reinvented wheel.)
Introduce Purl.  #18.

We'll use this for parsing hostnames out of archive URLs.
Display archive URLs in job history.  #18.