Added the initial matrix contracts file with a sample pre- and postcondion

@bsoetaer 
In any rational language, you're right - however, that logic doesn't hold in ruby.  In this case, `each_with_index` is referencing an implicit `self`.  If this module is `include`d in a class, and the precondition function is called from that class - in this case, `Matrix` or some variant thereof - then `self` will refer to an instance of `Matrix`.

That's a convenient fact for us, because it means we don't need to actually pass the matrix itself into all the conditions / invariants.