Parameterize theme_countdown()

Refactoring the countdown theme to accept the timestamp, accuracy and event name as parameters will make it possible for site developers to create more complex blocks with the countdown timer as a component.

This patch does not move the PHP logic out of the tpl.php file and into a preprocess() function as #1194950: Move PHP code from template to preprocess function suggests, but that should be done, too. This is the minimum needed to parameterize the template.