Syllogism demo

Initial pull request for testing syllogisms. Work in progress. Not to be merged yet. Comments welcome.
Thanks for the note, Cosmo. I wanted to reuse what already exists, i.e. the client API, but as these tests test PLN, I figure that they should stay in the pln repo. Maybe it'd make sense to put the client API in the opencog repository if it's going to be used more extensively in the future?
Sure. An extension is feasible and is what I had in mind. So you would suggest to run the entire demonstration in the CogServer using the client API with ```agents-step``` etc? I thought about this but shied away from this originally because I liked the ability to print changes directly to stdout and the simplicity of monitoring and checking with Python in contrast to reasoning in the CogServer and then having to load everything back into Python to check for the expected outputs. In the context of actually getting the syllogisms to work, I figured doing it directly in Python would be more straight-forward and educational. What do you think? Since everything's is going to be ported to the atomspace later anyway, we might as well use it already for testing?