naive Sphinx documentation

- translated all docstrings from Markdown to ReST
- added trivial documentation configuration
Why not just use

```
.. automodule:: pysoundfile
    :members:
    :undoc-members:
```
?

Then we could manipulate the order by re-ordering the source code (see #57).
This would need `autodoc_member_order = 'bysource'` in `conf.py`.