sframe Cannot create sarray of value type <type 'unicode'>

The testing code is below:

``` python
import sframe as sf
t = sf.SFrame({"id": ['a', 'b', 'c'], 'value': [1, 2, 3]})
t['str'] = 'aaa'
t['uni'] = u'aa'
```

Will sarray.from_const support unicode value?
