Fix pretty reprs of super() objects

[takluyver]

Prompted by this SO question.

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>

[ellisonbg]

@takluyver the code looks fine here, but the tests are failing with a probably unrelated JS test. Do you want to rebase on master to see if that will pass, or should we just merge and pray?

[takluyver]

We're seeing JS test failures quite frequently at the moment - I think @jdfreder and @minrk are working on making that better. However, not much prayer is needed in this case - there's no way that the JS tests are going to be affected by the representation of super objects.

[ellisonbg]

OK, I think all the PR related to test stability are in, so I am going to merge this one. If there is a problem in master we can clean it up...