Prepare the environment of Sphinx documentation (Issue 138)

http://trac.xapian.org/ticket/138

- Preparing the environment
- Integrating "getting started" documentation within it
- Automating the version 
- Link it to the global building system
- Apply the same on python 3

You can browse how it looks here: http://xapian.alfanous.org/

what's next? 
- Check building doc errors
- An extension for sphinx to include function declared by assignment

```python
MultiValueKeyMaker.add_value = new_instancemethod(_xapian.MultiValueKeyMaker_add_value,None,MultiValueKeyMaker)
```

Dependencies:
python-sphinx