Upload pysubnettree to pypi

It would be really awesome if pysubnettree were packaged on pypi.  Other python libraries could depend on it and have it automatically installed.  I modified the build code so that it will work with setuptools.  Once you're set up on pypi all you need to do is:

```
python setup.py sdist upload
```

You might also want to fill out a few more metadata fields in setup.py.