packaged for standalone installation (Python 3 and Python 2)

Hi,

In case you're interested, I packaged PROPKA 3.1 with [setuptools](http://pythonhosted.org/setuptools/) so that it can be installed as a standalone tool (named `propka31` instead of `propka.py` to avoid some naming/import problems) using standard python mechanisms:

    python setup.py install

or with [pip](http://www.pip-installer.org/en/latest/)

    pip install <tarfile>

I also added the Schrödinger patches so that it works with Python 2.7. All tests still pass.

If you wanted to, you could upload it to PyPi so that people can just install it from the internet by saying

    pip install PROPKA==3.1

(or similar). The metadata in `setup.py` is incomplete (e.g. not all authors, licence is unknown, etc) but it would not be much work to get it ready for `python setup.py upload`.

Thanks for making a great tool publicly available.

Best wishes,
Oliver