BUG - to_excel() fails using xlsxwriter if the index uses local timestamps
``` python
In [93]: ts= pd.Timestamp(datetime.datetime(2014, 1,1)).tz_localize(
    ...:     'America/New_York')
    ...: df=pd.DataFrame([1],index=[ts])
    ...: df
Out[93]: 
                           0
2014-01-01 00:00:00-05:00  1

In [94]: df.to_excel('/home/yank/2.xls')
TypeError: can't subtract offset-naive and offset-aware datetimes
```

I'm using 0.15RC1, dateutil: 2.2, pytz: 2014.7 and xlsxwriter 0.5.5 all fresh pip.
