Install the runner

It's time to officially release the runner (and its great [reports](http://stb-tester.com/runner.html))
to the world.

--

All the tests pass, except `test_stbt_batch_run_sigint_once` and
`test_stbt_batch_run_sigint_twice`.

Possibly because the old process tree used to look like this:

    extra/runner/run
    ↳ stbt run

But now it looks like this (when run from the unit tests):

    stbt-batch run
    ↳ stbt-batch.d/run
      ↳ stbt run

When run interactively and you press Control-C, it works as expected
(presumably because the shell sends the signal to the entire process
group).