Cache: Use Cache Group instead of 'prefix'
Cache: Simplify cache names
Cache: Utilize Cakephp's Cache Group feature

This simplifies cache management and especially clearing the cache
in CachedBehavior. Instead of looping through an array of registered
prefix, just call clearGroup().
Cache: Add missing DS
Cache: Rename config to 'cached_settings'
Croogo: Cache event handlers
Acl: Update cache config/group name
Merge branch '1.5-cache-groups' into 1.5