BUG: indexing with an entire period_index raising
```
In [57]: pi = pd.period_range('2002-01','2003-12',freq='M')

In [58]: pi
Out[58]: 
<class 'pandas.tseries.period.PeriodIndex'>
freq: M
[2002-01, ..., 2003-12]
length: 24

In [59]: p = pd.Panel(randn(24,5,10),items=pi)

In [60]: p
Out[60]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 24 (items) x 5 (major_axis) x 10 (minor_axis)
Items axis: 2002-01 to 2003-12
Major_axis axis: 0 to 4
Minor_axis axis: 0 to 9
```

This is ok

```
In [61]: p.ix[list(pi)]
Out[61]: 
<class 'pandas.core.panel.Panel'>
Dimensions: 24 (items) x 5 (major_axis) x 10 (minor_axis)
Items axis: 2002-01 to 2003-12
Major_axis axis: 0 to 4
Minor_axis axis: 0 to 9
```

This raises?

```
In [62]: p.ix[pi]
DateParseError: day is out of range for month
```
