fixed the stopwatch slowing problem

If your switch your tab in your browser while it has to interpret a lot of JS code, the setInterval() callback won't be called just once in a second. This always happens in Safari browser, because it always tries to save laptop's energy and gives to the scripts from other tabs a very low priority. So, I replaced the incremental function with comparing the current timestamp and time when the stopwatch was started. This should make the stopwatch more precise