Add images

Note: These images are not owned by me, and are not included in the license of
this tutorial. They are from
http://www.wolframcdn.com/sponsor-ads/Mathematica-ring-a.png and
http://www.qwantz.com/index.php?comic=1013.
Add some more exercises
Merge branch 'master' of github.com:certik/scipy-2013-tutorial