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