stbt batch summary chart: Logarithmic y axis (test duration)

When a freak test ran for 10 (!) hours, every other test ended up flat
against the bottom of the chart, so the chart was next to useless. With
a logarithmic scale, the chart is useful when mixing long- and short-
running tests in the same report.
Merge pull request #229 from drothlis/log-y-axis

stbt batch summary chart: Logarithmic y axis (test duration)