Implement generic List properties

@jpsim 
If min and max are generic, shouldn't sum also be? The sum of some integers is still an integer, so it would be nice if it returned it _as_ and integer. I suppose then we would need some kind of protocol that includes Ints, Floats and Doubles. http://stackoverflow.com/questions/24073135/apple-swift-generic-type-constraint-for-numerical-type-only