Respect numpy fixed length strings
pandas converts all strings to 'O' columns.

For consistency and writing to files it would be good if pandas respects numpy fixed length strings.

```
import numpy as np
import pandas as pd

df = pd.DataFrame({'date': np.array(['2015-10-10', '2015-01-01'], dtype='S10')})
print(df.date.dtype)
```

Results in `dtype('O')` and not `dtype('S10')`
