* Migrated the caches to return `NSSet` to eliminate duplicate objects when your cache keys overlap
* Introduced new recursive strategy for building cache keys in `RKInMemoryManagedObjectCache`
* Added support for array cache key values in `RKFetchRequestManagedObjectCache`
* Re-enabled a slew of tests that were disabled during 0.20 development
* Replaces primary key with `RKEntityIdentifier`
* Add support for use of compound keys for object identification
* Refactor `RKConnectionMapping` to `RKConnectionDescription` and add support for connecting with multiple attributes
* Clarify naming of representation key methods to better match naming conventions
* Add type transformation support for object identification
* Greatly expand test coverage for object identification
* Drop the `NSEntityDescription` category
* Simplify the `RKManagedObjectCaching` protocol
* Add compound key support to the Fetch Request and In Memory Cache implementations
* Replace Kiwi with Specta for tests where contexts are helpful for organization
* Rename `defaultValueForMissingAttribute` to `defaultValueForAttribute`
classes to eliminate issues with duplicated objects. closes#611, #612, #613, #618
* NSEntityDescription is now aware of the primaryKeyAttribute. Can be configured via
Interface Builder within Xcode or programatically.
* Added findByPrimaryKey: interface to the Core Data extensions.
* Relaxed dependencies on RKManagedObjectMapping across the system now that primaryKey is
available without a reference to the mapping.
