Adding stopwatch and countdown classes for more precise timing while still keeping the original timer class for reverse compatibility.

stopwatch / countdown use the system clock. This enables them to persist through many scenarios, including system hibernate. Note that changes to system clock will effect active time counters.

$.stopwatch(function () {
    $("#some-ele").html(this.getFormattedTime());
});

$.countdown(function() {
    $("#some-ele").html(this.getFormattedTime());
    if (this.isFinished()) {
        alert("Countdown complete!");
    }
}, 20);

Original PR: #13