Add “infinite” mode

I (and [many](http://stackoverflow.com/questions/15803937/restart-countdown-timer-when-expired other) [others](http://stackoverflow.com/questions/15803937/restart-countdown-timer-when-expired)) would like timers that automatically reset themselves after expiry. An example use case would just be someone (like me) who wants to update a status page every 30 seconds. Sure, I *could* just use setInterval, but I like to show a little countdown that displays how long until the page auto-refreshes itself. I could also just restart the timer myself (outside the plugin), but I’d prefer for this functionality to be built in.

I added an `infinite` option that does just that. If you set `infinite` to `true` ***and*** you have an `until` option that is a relative time, then the timer restarts after it expires. If you have an `until` that is a date, infinite mode doesn’t really make sense, so nothing happens in that case.

**Disclaimer** - this code works awesome for me, but I have not tested it against all the possible configurations and uses.