extra/runner/server: Generate directory listings

If the requested path is a directory that doesn't contain an
`index.html` file, generate a directory listing, much like apache's
mod_autoindex.

This allows you to run `server` from one directory (or more) *above* the
directory where you ran `run` (i.e. the directory containing the testrun
results), to serve results from multiple different test "sessions".

I implemented the directory listing myself instead of using Flask's
AutoIndex plugin[1] because of deployment considerations: If I ever
choose to install `extra/runner` in the official stb-tester package I
don't want to be installing things like `flask-autoindex` with `pip`
(`flask-autoindex` isn't available in Ubuntu's package repositories, for
example). Also my implementation achieves much the same result with far
less code than Flask-AutoIndex.

[1] http://pythonhosted.org/Flask-AutoIndex/