Implement Jarno Virtanen's etag/last-modified example, since that causes even less load on the target server, if it supports them. Switched the db entries to hashes - even though we have to explicitly read and write them, that fits the commit model well enough, and lets us add new verbs later.
Also implemented "summary" verb which tells us that we have 57 entries, 26 have etags, 28 have last-modified times - so this change makes a difference on more than half of the comics sites...