Make irclog2html -o accept a directory name

Make it require a directory name when processing multiple files.

Add tests.

This completes the feature introduced in #7.
Update to latest bootstrap.py (from zc.buildout 2.2.5)
Make 'make coverage' work again

New tox doesn't work when run from buildout: it tries to use

  subprocess.call([sys.interpreter, '-m', 'virtualenv', ...])

and so doesn't see the virtualenv that buildout installed and carefully
added to sys.path.  As a workaround install virtualenv directly into the
virtualenv that was used to bootstrap buildout.

(I feel I would simplify my life so much if I stopped using buildout.
But I want `bin/test -pvc` to Just Work!)