Do not record dates of unpublished posts

This fixes a problem where dates of unpublished posts are still recorded in the archive at the respective year/month/day. To test generate a post with a unique year (e.g. 1970), set it to ``published: false`` and visit the /archive link (all as configured in punch-blog) and notice a link to the 1970 year that won't show any posts though.

Word of warning: I am fairly new to JavaScript/node.js so this patch and spec change may or may not properly address the problem.