Makefile: Install extra/runner

It is inconvenient that the full path to 'stb-tester.git/extra/runner' has
to be specified to execute runner, and the readme file has to be generated
manually from restructured text.

The command 'make install-runner' copies runner to a permanent location,
compiles README.rst and makes it available as 'man runner'.

The scripts 'run', 'report' and 'server' are installed to <prefix>/bin,
therefore they will be available as commands, without typing the full path.
extra/runner/report: Document the command line options

'report' has two command line options available, '--classify-only' and
'--html-only' that are useful for re-generating a report, but they were
not mentioned in the help message.
extra/runner/run, report: Reference 'man runner'

Mention in the help messages of run and report that runner has a man page
where users can find more detailed information (provided they installed
runner with 'make install-runner').