setup.py: Use setuptools-markdown

[msabramo]

Use setuptools-markdown to use README.rst for PyPI long description.

[brandjon]

Thanks for the suggestion.

This looks interesting, but it fails to render due to relative links in the readme. I've left you a feature request in return.

Before the next SimpleStruct release I'll strip the links, either manually or automatically, so the landing page on PyPI will be nice and tidy.