Syllogism demo

Initial pull request for testing syllogisms. Work in progress. Not to be merged yet. Comments welcome.
> Since everything's is going to be ported to the atomspace later anyway, we might as well use it already for testing?

The only thing that will be ported, is the Python PLN implementation...

I think it would be nice to do the whole demo using the client API, if you think that's feasible.

Keeping the client API separate is probably a good decision, for the purpose of encapsulation, because it doesn't depend on anything.

But, I do see the reasoning behind keeping the PLN demos in the PLN directory. I'm not quite sure what is the best course there.