Add a draft RPM specfile.

This follows <https://wiki.cam.ac.uk/ucs/Platforms_packaging_guidelines>
and expects that you'll be building a package like this:

python setup.py sdist -d .
rpmbuild -ta django-ucamwebauth-1.0.tar.gz

No clever version-number handling at present.
Merge branch 'master' of ssh://git.csx.cam.ac.uk/raven/django-ucamwebauth