Do not use fallback responseTime if responseTime is 0
Add comment on responseTime: 0 to README
Document responseTime: [min, max] option in README
Add more comments to README.
Add stronger validation for responseTime
Use default response time as fallback for invalid values
Merge pull request #214 from bdusell/issue-211-zero-response-time-v2

Merge fix for #211 into v2 branch