Move images to to a separate subdirectory

Makes the "presentation" directory less cluttered.
Move presentation back to repository root, add dependencies as submodules

* Move presentation back to repo root so index.html lives there and
  we can use GitHub project pages.
* Use submodules to handle dependencies on qrcode.js and reveal.js
  more elegantly.
* Drop Makefile and symlinks (no longer needed thanks to submodules).
Move markdown sources and custom CSS into separate directory

Further unclutter the repository root.
Update README to explain submodules

Remind users of git submodule {init,update}.