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]`.
I'm not sure we want these to throw actual `Error`s, that will halt test execution, and in certain cases it could cause the entire test suite to fail. Perhaps we just need to log this fact, and the fact that we're using the default value instead. If a developer isn't seeing behavior they expect, the `console` should be their first look. (And we can add that tidbit to the documentation.)