Create feature to add multiple asynchronous timers on a page.

[Ravenstine]

I re-organized and re-wrote parts of the code to allow for multiple timers on the same page. This is done by creating multiple timer objects that are based on the CSS selector of the elements used for the timers. I'm not sure if this is the best way to achieve this goal, but it still seems to work pretty well.

[chikamichi]

Hi,

Thank you for your PR. I am in vacation for the time being. I'll review this when I'm back home, which should be either next week or the week after.