Move or copy a bunch of content from README.rst to docs/

It's probably best if you just look at the diff for `README.rst` and compare it to [the preview](http://irskep-pillow.readthedocs.org/en/latest/index.html).

I also edited the installation instructions a tiny bit, and cut some of the cruft from `docs/conf.py`.

You can see how the new `README.rst` renders [here](https://github.com/irskep/Pillow/blob/doc-improvements/README.rst).