Add some more info on benchmarks.
Document intentions for the 'clean' command.

But remove it for now. It isn't implemented and it isn't crucial.
Move table list into imdb package.
Updating entities now wipes the entity tables but does not wipe the
atom/name tables.

In essence, the trade off made here is more convenient updating at the
expense of letting stale data linger in the database. The upside is
that stale data will not appear in search results since it will lack
a corresponding row in an entity table (movie, tvshow, episode, actor).
Better error checking on search directives.