Time interval should be calculated when calculating bandwidth speed

[xiaosongxiaosong]

https://github.com/muaz-khan/getStats/blob/master/dev/getStats.js#L39

        if (getStatsResult.audio && getStatsResult.video) {
            getStatsResult.bandwidth.speed = (getStatsResult.audio.bytesSent - getStatsResult.bandwidth.helper.audioBytesSent) + (getStatsResult.video.bytesSent - getStatsResult.bandwidth.helper.videoBytesSent);
            getStatsResult.bandwidth.helper.audioBytesSent = getStatsResult.audio.bytesSent;
            getStatsResult.bandwidth.helper.videoBytesSent = getStatsResult.video.bytesSent;
        }