Added option for startDate

Sometimes it may be useful to have an option for ``startDate``, for example if you want the countdown not to rely on the client's time but on the server time.

Example:
````
// get time from server
var req = new XMLHttpRequest();
var url = '/?time='+ new Date().getTime();
req.open('HEAD', url, false);
req.send(null);
var currentDate = req.getResponseHeader('Date');

var finalDate = '2014/12/10';
$('.countdown').countdown(currentDate, finalDate);
````