Some improvement to example notebook

split on heading cell to have anchor and link back to homepage.

This allow to have anchors both in live notebook and on [nbviewer](http://nbviewer.ipython.org/urls/raw.github.com/Carreau/iruby/fix-en/IRuby-Example.ipynb#Example), also it's a good practice as we will add collapse and things like that using heading cells as hint.