Fix pretty reprs of super() objects

Prompted by [this SO question](http://stackoverflow.com/questions/21531713/why-does-ipython-display-super-objects-differently-from-the-regular-python-int).

The problem:

```
In [1]: class A(object):
    pass
   ...: 

In [2]: class B(A): pass

In [3]: class C(B): pass

In [4]: c = C()

In [5]: super(B, c)
Out[5]: <super: __main__.C, <__main__.C at 0x2a772d0>>

In [6]: repr(super(B, c))
Out[6]: "<super: <class 'B'>, <C object>>"

In [7]: super(B)
Out[7]: <super: None, None>

In [8]: repr(super(B))
Out[8]: "<super: <class 'B'>, NULL>"
```

Afterwards:

```
In [6]: super(B, c)
Out[6]: <super: __main__.B, <__main__.C at 0x1fbb6d0>>

In [7]: super(B)
Out[7]: <super: __main__.B, None>
```