Add a theme function for the block content

This patch applies against version 5.x-1.0 and provides a theme function for all of the block content, simply by throwing most of what was in countdown_block's case: view into a new theme_countdown_block() function.

There might be more logic in this function than there should be, but it will allow people to copy it and put it in their template.php file as your_theme_name_countdown_block() or phptemplate_countdown_block() and completely customize the output-- plus if we do move some logic out of the theming portion and pass it in as variables, it shouldn't break anything for people using this method.

Demo http://dev.wsf2008.net -- all we've done is add a break, but Agaric's theming department will be able to get to it soon.

committed