Issue 211 zero response time

Pull request for #211. This discards the use of a fallback value of `50` for `responseTime` when the user supplies `0`. The interpretation of `0` to mean "shortest possible latency" is now explicitly documented in the README.

I avoided implementing unit tests as there is no reliable mechanism for distinguishing a `setTimeout` callback which was given 50ms from one which was given 0ms but was scheduled later.

I also took the liberty of documenting `responseTime: [min, max]`.
And it needs to be positive or zero...