to_excel function unable to subtract offset-naive and offset-aware datetimes
Pandas version: **0.15.2**

I have a dataframe object with one column of timestamps:

```
ts value
2012-01-01 00:10:00 100
2012-01-01 00:20:00 200
...
```

And when I dumped them into csv file using to_csv, they are stored properly.
While when I dumped them into xlsx file using to_excel. the data in file are miss detailed timestamps. like:

```
ts value
2012-01-01 00:00:00 100
2012-01-01 00:00:00 200
...
```

Another error message would is:

```
  File "C:\Anaconda\lib\site-packages\pandas\util\decorators.py", line 60, in wrapper
    return func(*args, **kwargs)
  File "C:\Anaconda\lib\site-packages\pandas\core\frame.py", line 1230, in to_excel
    startrow=startrow, startcol=startcol)
  File "C:\Anaconda\lib\site-packages\pandas\io\excel.py", line 813, in write_cells
    cell.val, style)
  File "C:\Anaconda\lib\site-packages\xlsxwriter\worksheet.py", line 62, in cell_wrapper
    return method(self, *args, **kwargs)
  File "C:\Anaconda\lib\site-packages\xlsxwriter\worksheet.py", line 381, in write
    return self.write_datetime(row, col, *args)
  File "C:\Anaconda\lib\site-packages\xlsxwriter\worksheet.py", line 62, in cell_wrapper
    return method(self, *args, **kwargs)
  File "C:\Anaconda\lib\site-packages\xlsxwriter\worksheet.py", line 675, in write_datetime
    number = self._convert_date_time(date)
  File "C:\Anaconda\lib\site-packages\xlsxwriter\worksheet.py", line 3454, in _convert_date_time
    return datetime_to_excel_datetime(dt_obj, self.date_1904)
  File "C:\Anaconda\lib\site-packages\xlsxwriter\utility.py", line 617, in datetime_to_excel_datetime
    delta = dt_obj - epoch
  File "tslib.pyx", line 752, in pandas.tslib._Timestamp.__sub__ (pandas\tslib.c:14192)
TypeError: can't subtract offset-naive and offset-aware datetimes
```

A quick fix is to downgrade the version from **0.15.2** to **0.14.0**. It will dump the data correctly.
If it already fixed in the later version, just ignore that.
