io: to_excel leads to TypeError when index with tz
I would like to save a _df_ with the following _index_ to an Excel file:

```dtype='datetime64[ns, Atlantic/Reykjavik]', name=u'Dates', length=4360, freq='H')

```

it shows the following error:
```

TypeError: Timestamp subtraction must have the same timezones or no timezones

```
```
