Series with MultiIndex with PeriodIndex can't do unstack()
Here is the code:

``` python
import pandas as pd

idx1 = pd.PeriodIndex(["2013-01", "2013-01", "2013-02", "2013-02"], freq="M")
idx2 = ["a","b","a","b"]
value = [1,2,3,4]

idx = pd.MultiIndex.from_arrays([idx1, idx2])
s = pd.Series(value, index=idx)

s.unstack()
```

It will raise following exception:

```
anaconda/lib/python2.7/site-packages/pandas-0.12.0.dev_424e187-py2.7-linux-x86_64.egg/pandas/tseries/period.pyc in _from_arraylike(cls, data, freq, tz)
    649 
    650                 if freq is None:
--> 651                     raise ValueError(('freq not specified and cannot be '
    652                                       'inferred from first element'))
    653 

ValueError: freq not specified and cannot be inferred from first element
```
