unexpected behaviour when reindexing a Panel with MultiIndex
 I was trying to reindex a Panel with a MultiIndex. The ultimate goal is to be able to do slicing like this:

```
wp.loc[(1,slice(None)),:,:] #Access the panel fixing the first index of the multiIndex
```

But whenever I try to reindex, like in this example I got this strange behaviour.

```
wp = pd.Panel(pd.randn(4, 5, 4), items=['Item_1_1', 'Item_1_2','Item_2_1','Item_2_2'],major_axis=['a','b','c','d','e'],minor_axis=['A', 'B', 'C', 'D'])
a=[tuple([int(i) for i in item.split('_')[1:]]) for item in wp.items]
swp=wp.reindex(items=x)
```

But when I try to access an item of the panel I got;

```
In [101]: swp[1,1]
Out[101]: 
    A   B   C   D
a NaN NaN NaN NaN
b NaN NaN NaN NaN
c NaN NaN NaN NaN
d NaN NaN NaN NaN
e NaN NaN NaN NaN
```

The current workaround I have found is the following:

```
def sp(string):
    s=string.split('_')
    return int(s[-3]),int(s[-1])

 wwp=wp.rename(items=sp)
 w=wwp.items.sortlevel(level=0)
 swwp=wwp.reindex(items=w[0])
```

I don't think that the first is something that is excepted, and the second way is quite complex for nothing, but I might be wrong. 
And now it is working at it should. 
