python3 compatibility

- Things are suggested in python3 porting guide.
https://docs.djangoproject.com/en/1.8/topics/python3/

1. adding ```from django.utils.encoding import
python_2_unicode_compatible```

2. ``` __str__``` instead of ```__unicode__```
https://docs.djangoproject.com/en/1.8/topics/python3/#str-and-unicode-methods

3. Adding ```from __future__ import unicode_literals``` at the top
of your Python modules
https://docs.djangoproject.com/en/1.8/topics/python3/#unicode-literals

4. Removing the `u` prefix before unicode strings;
https://docs.djangoproject.com/en/1.8/topics/python3/#unicode-literals

- also closed #66

Signed-off-by: Hiroshi Miura <miurahr@linux.com>