Generate UUIDs for ArchiveUrl objects.  #18.

Motivation: CouchDB storage.  That said, they're generally useful.
ArchiveUrl: add type field to JSON form.  #18.

This is convenient for the CouchDB form of an ArchiveUrl, and is also
just a convenient habit to get into.
Fix logic glitch.  #18.
Don't produce ArchiveUrls that fail #valid? test.  #18.
Date munging.  #18.
Wire up ArchiveFinder.  #18.