Set Field.empty_values in case is does not exist

Fixes #10.

Django 1.6 introduces the attribute Field.empty_values (see https://code.djangoproject.com/ticket/19997 and https://github.com/django/django/commit/4cccb85e292fea01b3459cd97d751ed35179a7b7). Using this new attribute would break support for earlier Django versions.

This fix checks the existence of empty_values during init. If it does not exist (in case of Django < 1.6) it will be created in the exact same way as Django 1.6+ does.