to_excel with engine='xlwt' doesn't support pandas.tslib.Timedelta
Hello,

`to_excel` method with `engine='xlwt'` doesn't support `pandas.tslib.Timedelta`

```
import numpy as np
import pandas as pd

idx = pd.date_range('20140101', '20140201')
df = pd.DataFrame(index=idx)
df['col0'] = np.random.randn(len(idx))
s_idx = pd.Series(idx, index=idx)
diff_idx = (s_idx-s_idx.shift(1)).fillna(pd.Timedelta(0))
df['diff_dt'] = diff_idx
df.to_excel("out.xls", engine='xlwt')
```

it works fine with xlsx files (engine `openpyxl` or `xlsxwriter`).

Kind regards
