SQLAlchemy string types - encoding and fixed lengths

```Python
 In [1]: from blaze import *

In [2]: from blaze.data.sql import dshape_to_alchemy

In [3]: dshape_to_alchemy('string')
Out[3]: Unicode()

In [4]: dshape_to_alchemy('string[40, "ascii"]')
Out[4]: String(length=40)
```