naive Sphinx documentation

- translated all docstrings from Markdown to ReST
- added trivial documentation configuration
It is indeed a nice summary with nice examples.

I guess I was mainly disturbed by the listing of the endian-nesses, which is repeated below (and not that important).

What about this?

```
        * an *endian-ness*, which doesn't have to be specified at all in
          most cases (because the default ``'FILE'`` should do fine).
```

Maybe the thing in parentheses is not even necessary.

BTW, we should mention `default_subtype()` in the "Parameters" section!