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.

Before: 
![before](https://cloud.githubusercontent.com/assets/12424/4774768/8b1b7254-5bb0-11e4-8eef-73534bb49614.png)

After:
![after](https://cloud.githubusercontent.com/assets/12424/4774771/91e4e7f0-5bb0-11e4-9328-c446b686855d.png)