Update Pythran documentation

* Fix several typos / mistakes
* (Manually!) cleanup logo
* Update titles and rendering of several blocks
* Move to Sphinx for static website generation.
Centralize version number

It now lies in __init__.py as __version__, as suggested by

http://legacy.python.org/dev/peps/pep-0396/

setup.py and sphinx load it there now.

Thanks to Pierrick for the tip!