Prepare the environment of Sphinx documentation (Issue 138)

http://trac.xapian.org/ticket/138

- Preparing the environment
- Integrating getting started documentation within it

It looks now like this:
![screenshot from 2014-04-21 14 07 50](https://cloud.githubusercontent.com/assets/315228/2754502/68ae32ca-c956-11e3-8f71-1d5538b0222d.png)

What's next?
- Check building doc errors
- Automating the version 
- Link it to the global building system
- Highlighting python code  indside documentation

Dependencies:
python-sphinx 
```sh
pip install sphinx
```