Generate idents off of normalized URIs.

While there is value in having Job#url return a direct string
representation of #uri[1], it is also true that http://www.example.com
and http://www.example.com/ are actually the same identifier.
The hash-ident should reflect that.

[1]: Sort of.  I think replacing Job#url with job.uri.to_s is even
     better, but we can do that in a separate commit.