Add logic to "split" a split at a certain time, to create another split

[dpfens]

StopWatch.prototype.splitSplit = function(index, timestamp) {
    return newSplit;
}

This method should be called something that denotes that in order to add a split, that time must be taken from a pre-existing split to make room for it,

[dpfens]

instead added a method addRelativeSplit to add a split to the stopwatch at a given number of milliseconds since the start of the stopwatch.