to_excel can't handle index with time zones
Hi

Something is wrong when trying to save a dataframe with tz-aware timestamps to xlsx, using pd.Dataframe.to_excel:

``` Python
df = pd.DataFrame([1], index=[pd.Timestamp('2014-05-02', tz='CET')])
df.to_excel('test.xlsx')
```

yields an exception on my system:

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-4e18a4be2a71> in <module>()
----> 1 df.to_excel('test.xlsx')

/home/silvio/prod34/lib/python3.4/site-packages/pandas/core/frame.py in to_excel(self, excel_writer, sheet_name, na_rep, float_format, cols, header, index, index_label, startrow, startcol, engine, merge_cells)
   1202         formatted_cells = formatter.get_formatted_cells()
   1203         excel_writer.write_cells(formatted_cells, sheet_name,
-> 1204                                  startrow=startrow, startcol=startcol)
   1205         if need_save:
   1206             excel_writer.save()

/home/silvio/prod34/lib/python3.4/site-packages/pandas/io/excel.py in write_cells(self, cells, sheet_name, startrow, startcol)
    771                 wks.write(startrow + cell.row,
    772                           startcol + cell.col,
--> 773                           cell.val, style)
    774 
    775     def _convert_to_style(self, style_dict, num_format_str=None):

/home/silvio/prod34/lib/python3.4/site-packages/xlsxwriter/worksheet.py in cell_wrapper(self, *args, **kwargs)
     55             if len(args):
     56                 int(args[0])
---> 57             return method(self, *args, **kwargs)
     58         except ValueError:
     59             # First arg isn't an int, convert to A1 notation.

/home/silvio/prod34/lib/python3.4/site-packages/xlsxwriter/worksheet.py in write(self, row, col, *args)
    374         # Write datetime objects.
    375         if isinstance(token, date_types):
--> 376             return self.write_datetime(row, col, *args)
    377 
    378         # Write number types.

/home/silvio/prod34/lib/python3.4/site-packages/xlsxwriter/worksheet.py in cell_wrapper(self, *args, **kwargs)
     55             if len(args):
     56                 int(args[0])
---> 57             return method(self, *args, **kwargs)
     58         except ValueError:
     59             # First arg isn't an int, convert to A1 notation.

/home/silvio/prod34/lib/python3.4/site-packages/xlsxwriter/worksheet.py in write_datetime(self, row, col, date, cell_format)
    666 
    667         # Convert datetime to an Excel date.
--> 668         number = self._convert_date_time(date)
    669 
    670         # Add the default date format.

/home/silvio/prod34/lib/python3.4/site-packages/xlsxwriter/worksheet.py in _convert_date_time(self, dt_obj)
   3265     def _convert_date_time(self, dt_obj):
   3266         # Convert a datetime object to an Excel serial date and time.
-> 3267         return datetime_to_excel_datetime(dt_obj, self.date_1904)
   3268 
   3269     def _options_changed(self):

/home/silvio/prod34/lib/python3.4/site-packages/xlsxwriter/utility.py in datetime_to_excel_datetime(dt_obj, date_1904)
    577 
    578     # Convert a Python datetime.datetime value to an Excel date number.
--> 579     delta = dt_obj - epoch
    580     excel_time = (delta.days
    581                   + (float(delta.seconds)

/home/silvio/prod34/lib/python3.4/site-packages/pandas/tslib.cpython-34m.so in pandas.tslib._Timestamp.__sub__ (pandas/tslib.c:11918)()

TypeError: can't subtract offset-naive and offset-aware datetimes
```

Using Python 3.4.0 on Arch Linux, list of installed packages:

```
~% pip freeze
Cython==0.20.1
Jinja2==2.7.2
MarkupSafe==0.21
Pygments==1.6
SQLAlchemy==0.9.4
XlsxWriter==0.5.3
ecdsa==0.11
ipdb==0.8
ipython==2.0.0
ipython-sql==0.3.1
lxml==3.3.5
matplotlib==1.3.1
mysql-connector-python==1.1.6
nose==1.3.1
numpy==1.8.1
openpyxl==1.8.5
pandas==0.13.1
paramiko==1.13.0
patsy==0.2.1
prettytable==0.7.2
psycopg2==2.5.2
pycrypto==2.6.1
pyodbc==3.0.7
pyparsing==2.0.2
python-dateutil==2.2
pytz==2014.2
requests==2.2.1
scipy==0.13.3
six==1.6.1
sqlparse==0.1.11
statsmodels==0.6.0
tornado==3.2
xlrd==0.9.3
```
