Setuptools

I added a `setup.py` so that this can be installed system wide. I also created a package of it in my [Ubuntu PPA](https://launchpad.net/~martin-ueding/+archive/stable) with that (`python3-linfit-2014.05.30-0ubuntu1`). This required that I convert your single module `linfit.py` into a package `linfit`. That should not hinder the merge into NumPy at all, since the actual file is not changed.