This is a simple countdown plugin which displays a countdown effect.
The usage could't be simpler, just one line. For example:
$('span.countdown').countdown({seconds: 30});
You can add an option called 'callback', like this:
$('span.countdown').countdown({seconds: 30, callback: 'hello_world()'});
Then the function 'hello_world()' will be excuted when time out.
