[ADD] website: new countdown widget

- Task: https://www.odoo.com/web#id=8287&view_type=form&model=project.task
- Pad: https://pad.odoo.com/p/r.0coHaMolBpsxrIkg
@rim-odoo According to ECMAScript specification ( http://www.ecma-international.org/ecma-262/5.1/#sec-15.9.1.1 ), Javascirpt ignores the leap seconds. Also most of the computer-systems synchronizes time with help of NTP, and they do not show the case where seconds == 60(seconds will never be displayed as HH:MM:60), but they halt for a second, or they increases time continuously during the leap second, and then jumps back by 1 at the end of the leap second ( http://en.wikipedia.org/w/index.php?title=Network_Time_Protocol&section=7#Leap_seconds ) . So I think we can ignore this case. Kindy provide your feedback.