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]`.
If `min` and/or `max are not numbers this returns `NaN`, I'd prefer it to be `null` as it was before unless you have a good argument for `NaN`. Also, we can get negatives here pretty easily. If we're going to work on this line, can we add code to account for that?