Django-choices-flow

- Update default error message from Django settings
- Add customer messagem on Choice using 'error_msg' as restrict word
- add suport for django 1.6.x

- best performance for py3 and py2
- add suporte for group-choices
- fixed tests with py2 and py3

- suport for python3
- settings with custom error message

- add suport for south

- add django tests
- add model choices flow
- add choices flow
- add docs