Add inheritance-diagram to Sphinx documentation

Like this:

![inheritance-298b2ffbd7b15206a419cc17082e3b0da9c1c287](https://f.cloud.github.com/assets/29282/189983/f7fdd76a-7ebf-11e2-8f71-d3d877af6a20.png)

You need graphiz to build the doc.  Readthedocs support it.