to_excel doesn't support dateutil.relativedelta
Hello,

to_excel doesn't support `dateutil.relativedelta`

```
import pandas as pd
from dateutil.relativedelta import relativedelta
df = pd.DataFrame([relativedelta(years=2)])
df.to_excel('out.xls')
```

raises `Exception: Unexpected data type <type 'instance'>`

```
df.to_excel('out.xlsx')
```

raises `AttributeError: relativedelta instance has no attribute '__float__'`

Not sure if it's a Pandas issue, an XLS writer issue or a dateutil issue.
