Add `setup.py` file

`setup.py` is "a setuptools-based script for installing NailGun."

Both source distributions (eggs) and built distributions (wheels) pass a quick
sanity test:

    $ virtualenv .env2 --python=python2
    $ source .env2/bin/activate
    $ pip install ~/code/nailgun/dist/nailgun-0.0.1.tar.gz
    $ python -c 'from nailgun import client; print(client.get("http://www.ichimonji10.name").status_code)'
    200

    $ virtualenv .env2 --python=python2
    $ source .env2/bin/activate
    $ pip install ~/code/nailgun/dist/nailgun-0.0.1-py2-none-any.whl
    $ python -c 'from nailgun import client; print(client.get("http://www.ichimonji10.name").status_code)'
    200