Add Manifest.in file

A README.md file is not automatically picked up during packaging.
This is especially problematic because the setup script setup.py
tries to open this file. The build was thus failing.

Add MANIFEST.in file to include .md files at the root.
Add requirements.txt file

The requirements file is useful for quick installation using pip.
Add tox support to manage testing.

We want to use tox to manage our testing,
because tox is awesome.