MEP10: adding numpydoc and activating autosummary

Here is the first step in implementing MEP10 (point 1, and part of point 3)

- Added numpydoc to the sphinx extension
- Added sphinx's autosummary
- Made the output a little prettier (I hope).

Here is the screenshot of a page of the API
![MEP10_after](https://f.cloud.github.com/assets/184798/71409/a3fc9ac6-5fe7-11e2-9398-f92de4cf11af.png)

I chose to ship numpydoc with matplotlib, instead of asking developpers to install it. That point is debatable.
Also, because of numpydoc, the documentation will not build for sphinx below 1.0.