Add setup.py for pip install

Adding a setup.py file lets users install directly from github with pip, either with this command:

    pip install -e git+https://github.com/demiurgestudios/gevent_openssl#egg=gevent_openssl

...or by adding `-e git+https://github.com/demiurgestudios/gevent_openssl#egg=gevent_openssl` to a requirements file.

Let me know if any of the credits/license stuff is wrong, or if there's anything else you'd like changed before merging.