This marks a major step towards centralizing some of the "spec" logic
and creating a single access point for them. Because `ImageSpecFields`
are just alternative interfaces for defining and registering specs,
they can be accessed and overridden in the same manner as other specs
(like those used by template tags): via the spec registry.
Somewhere along the line, a change got merged that stopped using the
receivers module. This re-integrates it and moves changes made to the
old receivers (static methods on ImageSpecField) to them.
This new feature gives the user more control over *when* their images
are validated. Image cache backends are now exclusively for controlling
the *how*. This means you won't have to write a lot of code when you
just want to change one or the other.
While this change means users can no longer specify their own filenames,
changing a property of a processor, for example, will now result in a
new image. This solves a lot of the previous invalidation issues.
