Object indexes on the items/info axis of Panel
I have objects in the items axis of a Panel, selecting by an object gets me an infinite recursion. 

``` python
import pandas as pd
import numpy as np

class Key:
    def __init__(self, name, id):
        self.name = name
        self.id = id

    def __repr__(self):
        return "Key(name={name}, id={id})".format(**self.__dict__)

names = ['dale', 'bob', 'frank']        
N = len(names)
items = [Key(name, id) for name, id in zip(names, range(1, N+1))]        

panel = pd.Panel(np.random.randn(N, 10, 10), items=items)
panel.ix[items[0]] # infinite recursion
super(pd.Panel, panel).__getitem__(items[0]) # works
```

It appears the issue is that the `Key` object isn't be counted as scalar in the `Panel.__getitem__`

``` python
 253     def __getitem__(self, key):
 254         if isinstance(self._info_axis, MultiIndex):
 255             return self._getitem_multilevel(key)
 256         if lib.isscalar(key):
 257             return super(Panel, self).__getitem__(key)
 258         return self.ix[key]
```
