support notes.

There is no s5-notes.html in you repository, so i put one from web and tweak the org-s5 to support "NOTES" drawer and export as notes block in html.

example usage:
#+OPTIONS: toc:nil

\* slide1
  :NOTES:
  notes a bout slide1.
  :END:

\* slide2
  :NOTES:
  notes a bout slide2.
  :END: