Support for numpy strings
This is more of a question than an issue:

why does Pandas use object type rather than the native fixed length numpy strings?

For example:

```
In [24]: np.array(["foo", "baz"])
Out[24]: 
array(['foo', 'baz'], 
      dtype='|S3')
```

but:

```
In [29]: pd.DataFrame({"a":np.array(["foo", "baz"])}).dtypes
Out[29]: 
a    object
dtype: object
```
