runner report: Replace percentage with confidence interval in summary

As you run more repetitions of a test you get a better idea of how
reliable the test is.  A percentage is useful to describe a failure rate
independent of the number of runs but doesn't include information on how
reliable that percentage is likely to be.

This commit replaces the percentages in the report with percentage ranges
called "confidence intervals".  This means that instead of 43/430 being
a failure rate of 10% it will read 7.5% - 13%.  This means that there is
a 95% chance that the "true" reliability of the test falls between 7.5%
and 13% rather than the potentially misleading 10%.

We use the Wilson Score Interval[1] to calculate the confidence interval.
This assumes a binomial distribution (e.g. test pass/fail results are
independent of each other).

Possible future work:

* Include a further breakdown in the report summary by test exit status
  reason.
* Include a Chi-Squared Test[2] for comparing the results with different
  filters applied to answer questions like "has reliability improved
  between these two versions of software?".
* Write tutorial documentation to help users understand the statistics
  and how to interpret test results.

See also:

* Website for calculating confidence intervals[3]
* Evan Miller's excellent "Statistical Formulas For Programmers"[4]

[1]: http://en.wikipedia.org/wiki/Binomial_proportion_confidence_interval#Wilson_score_interval
[2]: http://www.evanmiller.org/ab-testing/chi-squared.html
[3]: http://vassarstats.net/prop1.html
[4]: http://www.evanmiller.org/statistical-formulas-for-programmers.html
runner report: Minor wording/spelling fixes, wrap lines
Merge branch 'report-confidence-interval'

Closes #76.

* report-confidence-interval:
  runner report: Minor wording/spelling fixes, wrap lines
  runner report: Replace percentage with confidence interval in summary
make check-pylint: Set PYTHONPATH for tests/fake-irnetbox

`fake-irnetbox` worked correctly when run from `run-tests.sh`, because
`run-tests.sh` sets PYTHONPATH correctly; now the `make check-pylint`
recipe also sets PYTHONPATH so that pylint can find the `irnetbox`
module used by `fake-irnetbox`.
extra/stb-tester.spec: Relax pylint < 1.0 requirement

We want people to be able to install the stb-tester RPM on systems like
Fedora 19 that have pylint 1.0. They just won't be able to use the new
`stbt lint` tool (due to bugs with pylint, and possibly differences in
the pylint plugin API; see 97eada33 and 65a38380). Note that `stbt lint`
checks the pylint version and exits with a clear error message.
Bump stb-tester version to 0.17

Note that the version number in the RPM spec file was already set to
0.17 (in 96afe36d; we haven't released the RPM since then so we don't
need to bump the version or release numbers).