ImageSpecField does not work in abstract base class

[vesterbaek]

Signals are registered on the class name:
https://github.com/jdriscoll/django-imagekit/blob/develop/imagekit/models/fields/__init__.py#L95-100

When an ImageSpecField is defined in an abstract model, the signals never get triggered when the superclass is e.g. saved. I'm using the NonValidatingImageCacheBackend, and am hence quite dependent on the triggering of the signals.

[matthewwithanm]

This is definitely a problem, but I'm not entirely sure how we should approach it. It seems like a Django bug and there's currently an open ticket to address it. I'm open to suggestions though.