BUG/API: PeriodIndex doesn't support loc using partial string 
#### Code Sample, a copy-pastable example if possible

`DataFrame.loc` supports partial string selection when it has `DatetimeIndex`. But it can't work if index is `PeriodIndex`.

```
ddf = pd.DataFrame(np.random.randn(10, 3),
                   index=pd.date_range('2011-01-01', freq='H', periods=10))
ddf.loc['2011-01-01']
# OK, returns DataFrame using partial string selection

pdf = pd.DataFrame(np.random.randn(10, 3),
                   index=pd.period_range('2011-01-01', freq='H', periods=10))
pdf.loc['2011-01-01']
#0    0.114496
#1   -1.900340
#2    0.870142
# Name: 2011-01-01 00:00, dtype: float64
```
#### Expected Output

Should return `DataFrame` using partial string selection, like `__getitem__`.
#### output of `pd.show_versions()`

current master
