Fix Django 1.6 _has_changed() incompatibility

In Django 1.6, Django forms dropped the _has_changed() method
from their own widgets, and want to call it on fields instead.

Also includes a workaround for the Django 1.6 test runner not
finding django-selectable's tests (using the old test runner).
Merge pull request #110 from dpoirier/django_16_has_changed_method

Fix Django 1.6 _has_changed() incompatibility. Fixes #109