API/ENH: stata export should better support np.datetime64 based times
The following code exporting a dataframe with a timeseries index with np.datetime64 type

``` python
import numpy as np
import pandas as pd

timestamps = pd.date_range(np.datetime64('2013-01-01 11:00:00.123456'), periods=5, freq='123U')
dataframe = pd.DataFrame(np.random.randn(len(timestamps)), index=timestamps)
dataframe.to_stata('out')
```

throws a

```
ValueError: Data type datetime64[ns] not currently understood. Please report an error to the developers.
```

It would be very nice if np.datetime64 (and thus pd.Timestamp as well) would be supported here.
