Abstract models containing ImageSpecFields are not handled properly

One of my models is abstract and includes several `ImageSpecField` fields.  Calling `./manage.py generateimages` leads to an exception in `imagekit/specs/sourcegroups.py` at the line:

        for instance in self.model_class.objects.all():

The attached commit fixes this particular exception, but I don't think it addresses all the potential issues involved with abstract base classes.  For example, the generator registry only includes one entry corresponding to the abstract base; it may make sense to instead list it for all the descendants that correspond to non-abstract models.