common IPython prefix for ModIndex

So that not all is not classified under `I`

Cf http://ipython.org/ipython-doc/dev/py-modindex.html vs 
![capture decran 2013-12-11 a 16 37 08](https://f.cloud.github.com/assets/335567/1725009/a0186df0-6279-11e3-9a65-26a404579724.png)