get_form_errors handling form field __all__ unicode

Hey Chris,

When the key __all__ is included in `form.errors`, django-jsonit seems to not like the unicode version of the `__all__` field. I made a quick fix to handle the unicode case:

                if field is not '__all__' or field is not unicode('__all__'):
                    field = form[field].auto_id

Not sure if there's a need to handle the non-unicode case anymore, but including that second condition seems to allow django-jsonit to return the expected errors via the __all__ key.