Cabalize

Package versions are mostly as produced by cabal init on my
system (that is, more or less what I happened to have
installed).

Feel free to enlarge the dependency range for what you have
installed, as long as the code still works.
Document how to run demo

I needed to check the source to find the right URL, so I
documented it.
Declare the status as "early prototype" in README.md

Since the demo is still not ready for wide exposure, I added a
warning that this is still an early prototype. This will avoid
disappointing casual users who will go to the homepage and read
the README.md.
Add .gitignore