Created a Set Difference class

Now I'm not in favor of this method. What do think should be the universal set for `Naturals`, should it be `Naturals`, `Integers` or `Reals`? What it should be for FiniteSet having all elements as Natural numbers? Should we have it has `Naturals`. I don't think we can have a definitive answer for these questions. If we try to predict too much of what she is trying to then the situation can easily get confusing both for us and the users. If take very generalized sets as the universal set then it will be as good as not having it. I think it is good to assume user knows what he is trying to do, in this case the user will know in which domain she is working.