Set up wheel distribution support

The changes in setup.cfg will build a single py2 & py3 compatible wheel.

You'll need the wheel package install in your local env when running `setup.py publish` from now on (until it's bundled I guess).

Some resources:
* http://pythonwheels.com
* https://hynek.me/articles/sharing-your-labor-of-love-pypi-quick-and-dirty/