Allow FieldTracker to work in child models
Allow FieldTracker to work in child models

Fixes #57.

yeago/django-model-utils,yeago/django-model-utils,carljm/django-model-utils,timmygee/django-model-utils,carljm/django-model-utils,nemesisdesign/django-model-utils,timmygee/django-model-utils,patrys/django-model-utils,patrys/django-model-utils,nemesisdesign/django-model-utils