Syllogism demo

Initial pull request for testing syllogisms. Work in progress. Not to be merged yet. Comments welcome.
I noticed that some of the syllogisms , such as [this one](https://github.com/sebastianruder/opencog/blob/syllogisms/opencog/python/pln/examples/relex2logic/syllogisms/syllogism-boat.scm) or [some of these](https://github.com/sebastianruder/opencog/blob/syllogisms/opencog/python/pln/examples/relex2logic/syllogisms/medium-syllogisms.scm) provide different conclusions and only the correct one should be returned while the others should be refuted. I figured there should be an explicit check for these.