Change spec/source registry to generator/cacheable
Register cacheables as generators instead of items
Merge branch 'ik-next' into cacheables

Conflicts:
	imagekit/management/commands/warmimagecache.py
	imagekit/registry.py
Revert signal names
Add LazyGeneratedImageCacheFile
Separate source groups and cacheables.

This allows a sensible specialized interface for source groups, but also
for ImageKit to interact with specs using the generalized image
generator interface.
Fix source callbacks on strategies
Update warmimagecache to use new cacheable registry
Always use colon as segment separator
Improve generator id pattern matching

This behavior allows users to easy generate images by app, model, or
field.
Fix pickling error
Merge branch 'cacheables' into ik-next