Online demo via GitHub Pages

Hello,

this patch adds a link to the github.io page that contains the demo. This makes it easy to try `ext-imagenannotation` without running a local web server.

The hosting is provided by GitHub Pages. To enable GitHub Pages you should do

    git remote add gioele git://github.com/gioele/ext-imageannotation.git
    git checkout --orphan gh-pages
    git pull gioele gh-pages
    git push

After some minutes the demo will be available online at <http://faustedition.github.io/ext-imageannotation>.