BUG: infinite loop when column index includes periodindex and string
the following code creates an infinite loop in python:

import pandas as pd
cols = [pd.Period('1998', 'A-DEC'), pd.Period('1999', 'A-DEC'), 'test']  
df = pd.DataFrame(np.random.randn(10, 3), columns=cols)
df[[pd.Period('1998'), pd.Period('1999')]]

It appears to be because Index.get_indexer changes the index from an int type to an object and recalls get_indexer which then changes it back to an object.
