Setuptools magic

Hopefully this will make the packaging and bootstrap process easier, @jdkasten :)

I believe it's better to sort out packaging in the Python-pure way (which allows for more systems compatibility out of the box), and leave the rest up to specific distro package maintainers. Setuptools automatically resolves the dependencies and installs the `letsencrypt` script to the `$PATH`.

Also updated README to use `python setup.py install --user` which somewhat fixes "sudo" problems mentioned in #10.

`python setup.py install/develop` tested on current Debian unstable, though there are slight problems due to https://github.com/Demonware/jose/pull/1.