* Metadata mapping is implemented via an `NSProxy` object that stands in for the `sourceObject` of each `RKMappingOperation`
* Key paths that are prefixed with `@metadata.` are mapped against the `metadata` dictionary of the operation, else they hit the `sourceObject`.
* Metadata is implemented by merging a dictionary of data in from `RKObjectRequestOperation`, through to `RKResponseMapperOperation`, into `RKMapperOperation`, and all the way down into the `RKMappingOperation`, where it is available to each object. By convention, metdata is namespaced into subdictionaries.
1. Eliminates use of visitor in favor of aggregating mapping info during mapping. This avoids having to traverse the mapping graph.
1. Eliminate special case handling of cyclic key paths.
1. Eliminate issues with infinite recursion while traversing the mapping graph.
* 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`
