Deprecated StrAndUnicode replaced with python_2_unicode_compatible.

I just replaced this deprecated base class with class decorator, so it django 1.7 compatible. Although  in docs/_ext/djangodummy/requirements.txt required django version was changed from 1.3 to 1.5, it has not actually changed as HEAD had not been able to run with django lower than 1.5 (due to django.utils.six dependency).