Switch to posts_selection for our author page fix

This will help prevent 404s on author pages if the author has no posts.
By setting, the queried object correctly, we prevent WordPress from
sending 404 headers.

posts_selection is used because it's one of the latest actions called
during WP_Query's get_posts method.

related #27
Cache the false condition in `get_guest_author_by()`

If the false condition isn't cached, `get_coauthors()` can trigger some nasty queries on every page load because `get_coauthor_by()` will look to see if there's a guest author that should override a WordPress user. If there is no guest author profile, then CAP will again check each time `get_coauthor_by()` is used.
Merge pull request #116 from Automattic/cache-falsy-116

Cache the false condition in get_guest_author_by