Session15

Adding Iterator homework 
There's not a whole lot of point to defining a `__del__` special method here.  That method is only called when an object is unbound from it's last referent, and the internal state of the object is due to be recycled by garbage collection anyway.  I would be very careful about overriding the `__del__` special method on any classes.  I can be a recipe for reference loops and memory leaks.