Dataframe fancy indexing behaviour with Periodindex via strings
```
cols=period_range('2000',periods=2)
df=DataFrame([[1,2],[3,4]],columns=cols)
cols = map(unicode,cols)
df.loc[:,cols]

KeyError: "[[u'2000-01-01', u'2000-01-02']] are not in ALL in the [columns]"
```

@jreback , not sure whether to call this a bug.
