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]`.
`NaN`, `null`, and negative numbers all cause `setTimeout` to fire immediately, so the point is mostly moot, but if I were to provide a nonsensical value for `setTimeout` I would rather get an error than have it use this behavior silently.

Maybe something like this would be better?
```javascript
        function isPosNum(value) {
            return typeof value === "number" && value >= 0;
        }

        function parseResponseTimeOpt(responseTime) {
                var result;
                if ($.isArray(responseTime) && responseTime.length === 2) {
                        var min = responseTime[0];
                        var max = responseTime[1];
                        if(isPosNum(min) && isPosNum(max)) {
                            result = Math.floor(Math.random() * (max - min)) + min;
                        }
                } else if(isPosNum(responseTime)) {
                        result = responseTime;
                }
                if(result == null) {
                        throw new Error('invalid responseTime');
                }
                return result;
        }
```