Added Python 3 packaging

This is the basic packaging for debian and ubuntu.

To build a package, run

    dpkg-buildpackage -S -tc  # to create a source package for a PPA
    dpkg-buildpackage -S -tc  # binary package

And to make sure the package has no errors / warnings:

    lintian ../*.deb ../*.changes ../*.dsc

For Python 2, you could simply replace `python3` with `python`.

NB: the version number for the package is defined in `debian/changelog`. It's best to check out a tag before building a package, if you plan on uploading it somewhere. Obviously, that tag doesn't exist yet.