Added SatisfyingSetToConceptRule

I added the SatisfyingSetToConceptRule as discussed in https://github.com/opencog/opencog/issues/613. It employs a new link type, the ConceptToPredicateLink, which is not yet recognized. I don't want to mess anything up so I'd rather ask where I would specify such a new link type. Is it sufficient to specify it in the types class in opencog.atomspace? If so, what number would I then assign it? This shouldn't be merged yet, as I'm still going to implement a unit test to check that the rule works as it's supposed to.