Fix the back link when the last item is an image
Remove asciicast embed script

Asciicasts can be embedded by just adding the html manually:

 #+BEGIN_HTML
 <script src=https://asciinema.org/a/###.js id=asciicast-### async></script>
 #+END_HTML
Add download links and generate PDFs

This includes the following changes
- add pdf/org-mode source download links to both the index page and
individual posts' pages
- Refactor the coffeescript so it's easier to read/maintain, and use the
new stuff to toggle the download links
Minify html

Also:
- Use $@ over the literal "index.html" in index.html's recipe