Changed the way post_save and post_delete signals are being handled.
Changed the way post_save and post_delete signals are being handled.

One handler is created per model instead of per bound image spec.
This cuts down on the number of handlers created, and also offloads the
policing of the handlers in memory to the signal framework. Since they are no
longer being created per spec, the handlers can be weakly referenced.

FundedByMe/django-imagekit,pcompassion/django-imagekit,FundedByMe/django-imagekit,pcompassion/django-imagekit,tawanda/django-imagekit,pcompassion/django-imagekit,tawanda/django-imagekit