Use setuptools console script for better windows support.

[rmcgibbo]

This is a fix for #34. It does a couple other things too.

• Remove requirements.txt and consolidate with install_requires. For PyPI packages, using install_requires makes more sense. https://caremad.io/2013/07/setup-vs-requirement/
• The setuptools console_script entry point works correctly on windows, whereas distutils scripts does not.
• Add a -v version command, because why not.