New Revealjs backend for asciidoctor

This is a new backend for Asciidoctor project based on RevealJS and using SLIM as Template Engine

The backend can be used and also tested with project `hyla` if you follow this instruction :

    git clone git@github.com:cmoulliard/hyla.git
    cd hyla
    gem build hyla.gemspec
    gem install hyla-1.0.2.pre.gem

    # Delete previously directory/project created
    rm -rf ~/hyla/MyRevealSlideShow
    cd ~/hyla

    # Create a new blank project for the slideshow
    hyla new --blank MyRevealSlideShow

    # Create an asciidoc file slideshow_revealjs.ad in this project
    hyla create --t slideshow --a revealjs --d MyRevealSlideShow

    # Generate HTML content
    cd MyRevealSlideShow
    hyla generate --backend revealjs --s . --d generated_content --r adoc2slide

    # Start local HTTP Server
    hyla serve -P 4000 -H localhost -b /hyla/ --out_dir ~/hyla/MyRevealSlideShow/generated_content/

   # Open Project in your browser
   open http://localhost:4000/hyla/slideshow_revealjs.html

   # Enjoy !