[rake-make-haml] ```rake slides.html``` now builds slides

* The goal is to let you author slides in HAML in slides.haml, and
  then this rake task will convert the HAML into HTML, and later on
  we'll splice that right into the reveal-js slides.

* Believe I've got the Rakefile setup so that slides.html is rebuilt
  any time slides.haml changes.
Merge branch 'rake-make-haml'