readme: translate into reST format

This will allow us to include the README as the long description in
the setup.py file. This will in turn make the README show up on PyPI
when we upload it there.

This is for #121.
setup: read long description from README

Working towards #121.
setup: avoid depending on third-party setuptools project

The stdlib distutils should be enough for our simple project.
readme: only mention "pip install zpm"

There is generally no need to explain how people can install from a
Git clone or use pip to install directly from GitHub -- these
instructions are the same for every project, and if they've already
found the README, they will know what to do next.