Fix issue 21

Fixes GH-21.

```python
# _test_u.py

from __future__ import absolute_import, division, print_function, unicode_literals

from pies.overrides import *

print(u('Bj\xf6rk Gu\xf0mundsd\xf3ttir'))
```

```
$ python3.3 _test_u.py
Björk Guðmundsdóttir

$ python _test_u.py
Björk Guðmundsdóttir
```