Add hatom formatting to templates

This should be pretty complete. We lost some of the classes we've been using throughout though, so check this over carefully. This may not be as backwards compatible as we thought, or in places where we lost the original class, we can add it as an additional class or try to add it to the `post_class()` function in functions.php.

On pages with lists of entries, `hfeed` is optional but might as well add it if we're going to all this trouble. :)

For each Entry, `hentry` is a required property, and `entry-title`, `author` and `updated` are all required sub-properties for a page/post to meet the hatom spec and pass Google's structured data testing tool. `entry-content` is also necessary (though technically optional, it is indicated in the spec that it "should" be there) in those instances where we have the text or part of the text; `entry-summary` could be used in posts lists, but it's not wrong and much easier to stick with `entry-content` for those too. In the cases when we do not wish to display the author, we've been using the site title.

If we do not wish to use both `updated` and `published` in the `time` element, it is acceptable to use only `published`. I wasn't sure which way to go on that, so I included both (which is perfectly valid and the preferred choice of some so-called "experts" on this).

I added `rel=bookmark` because the spec indicates each Entry *should* have an Entry Permalink. If we list linked Categories, those should include `rel=tag` if we are able to add that (assuming we use wp_list_categories() or something, we'll be a WP's mercy).