Update license to GPLv3
Update README, theme description
Category lists

Archive-type pages now display a list of posts, each with a thumbnail and excerpt, as per the discussion in #38. If the page is a category, it lists all subcategories at the top.
Major polishing

- Refactor and document stylesheets, `functions.php`, other files as per #31
- Tweak 404 and search failure pages
- Move `parts/content-templates/*` to `parts/*`
- Add empty alt tags to images missing them