Track downloaded URLs in the Lua script.

Wget seems to track downloaded URLs on a per-source URL basis, which
means that we end up downloading e.g. the same stylesheets, Javascripts,
etc. for each twitpic URL.  We only need to download those once per
WARC, so we implement our own URL tracking machinery.
Merge pull request #4 from ArchiveTeam/manual-tracking

twitpic.lua: track downloaded URLs in the Lua script