Puts <header> in intro.php, $post_status in post.php, adds CSS

This allows for the contents of `intro.php` to be styled separately than blog posts, and allows for different styles of blog posts. 

`$post_status` is now effectively a post class, allowing for different styles of posts: 
* normal (as `published`), 
* `draft` (hidden with `display:none in the CSS`)
* whatever else the theme author wants to make.