Noramlize 'setup.py' to fit conventions:

- Comply with flake8, except for indentation width, which is 2.

- Move human-centric metadata to top of 'setup()' call.

- Add Trove classifiers for supported Python versions.

- Use 'find_packages()' + MANIFEST.in to avoid errors in listing
  modules and packages.

Closes #131 and #161.