Panel partial index is transposed when minor_index is MultiIndex
I can index a Panel to get a data frame, e.g. by major_axis:

```
>>> p = pd.Panel(np.arange(8).reshape(2,2,2),items=('A1','B1'),major_axis=('A2','B2'),minor_axis=('A3','B3'))
>>> p.loc[:,'A2',:]

    A1  B1
A3   0   4
B3   1   5
```

But if I do the same index when the minor_axis is a MultiIndex the resulting data frame is unexpectedly transposed:

```
mi = pd.MultiIndex((['A3','B3'],['a','b']), [(0,0),(1,1)])
p = pd.Panel(np.arange(8).reshape(2,2,2),items=('A1','B1'),major_axis=('A2','B2'),minor_axis=mi)
p.loc[:,'A2',:]

   A3   
    b  b
A1  0  1
B1  4  5
```

Actually I think the second behaviour is closer to what I expect - the items axis becomes dimension 0 of the DataFrame.  But whichever is used, we should have stability, especially if 1-item MultiIndex objects automatically become Index objects.
