BUG: reset_index with a multi-index PeriodIndex
http://stackoverflow.com/questions/24718730/is-there-any-way-to-keep-a-periodindex-as-a-series-of-periods-with-a-reset-index

related: #7791, #7792 (and I think this causes both)
duped by #7793 

```
In [10]: df = DataFrame(np.arange(9).reshape(-1,1),index=pd.MultiIndex.from_product([period_range('20130101',periods=3,freq='M'),['a','b','c']],names=['month','feature']),columns=['value'])

In [11]: df
Out[11]: 
                 value
month   feature       
2013-01 a            0
        b            1
        c            2
2013-02 a            3
        b            4
        c            5
2013-03 a            6
        b            7
        c            8

In [12]: df.reset_index()
ValueError: Length of values does not match length of index
```
