ERR: raise on mutli-index columns as its not implemented in to_excel
Try to the following code, and you will find column labels mismatched with the data in the Excel file :

`cols = pd.MultiIndex.from_tuples([('site',''),('2014','height'),('2014','weight')])`
`df = pd.DataFrame(np.random.randn(10,3), columns=cols)`
`df.to_excel('test.xlsx', index=False)`

I am using pandas v0.16.0.
