Add setup.cfg to ensure package versions change during development

Without this all installs from git will have the version 0.1, even though 1.0-final is not out. This can cause setuptools to miss new commits as there is no bump in version number.

There is no native git support in distutils so we can't have a commit hash like subversion does, so the date is the best option as a false positive for a change is better than a false negative