a few improvement proposal


- Pin to Python 3 on recent version of IPython, 
   - You were already making some assumption about that (using annotation), so I went further and usef unicode variable  `sigma` in some place is now really the greek letter sigma.

- You mention the play button, I added an image of it, and linked to a longer introduction on IPython UI.

- Some examples were using the `--pylab` flag, which is deprecated, I went ahead, and made the import explicit. 

- I changed a bit some text, II've put David Ketcheson citation in a cite environment.

- Use mathjax for math formula instead of `<sup>`, and small mention you can use Latex. 

- fix scikit image example to work on recent version of scikit image, 

- link scikit image to their main website.