Use distutils instead of setuptools
Add descriptions

These are sort of important and should really have been included with
the initial commit, but they were accidentally not filled in.
Add MANIFEST to .gitignore

A MANIFEST file is generated during packaging now that distutils is
being used, but it doesn't need to be included in the repository.
Add platforms parameter to setup

It should work on any platform, so let specify that, especially because
it is a bit nicer than just having UNKNOWN in the package info.