Another major rewrite. Now BugDir, Bug, and Comment are more distinct.

Another major rewrite.  Now BugDir, Bug, and Comment are more distinct.

I pushed a lot of the little helper functions into the main classes,
which makes it easier for me to keep track of what's going on.  I'm
now at the point where I can run through `python test.py` with each of
the backends (by changing the search order in rcs.py
_get_matching_rcs) without any unexpected errors for each backend
(except Arch).  I can also run `test_usage.sh` without non-Arch errors
either.

However, don't consider this a stable commit yet.  The bzr backend is
*really*slow*, and the other's aren't blazingly fast either.  I think
I'm rewriting the entire database every time I save it :p.  Still, it
passes the checks. and I don't like it when zounds of changes build up.