* 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.
* Rename RKDynamicMappingMatcher to RKObjectMappingMatcher since it is not strictly coupled to dynamic mapping and works with object mappings.
* Rework matchers into using a class cluster style to enable flexible subclassing to introduce additional matchers.
* 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`
* RKObjectParameterization now returns an empty dictionary if mapping produces an unmappable representation error (no mappable attributes/relationships found)
* Log errors if request construction results in an NSError and return nil
* Ensure errors are passed back up through object parameterization instead of just returning nil
* Reset the serialization MIME Type registry during RKTestFactory set up to ensure it is in a sane state
* Convert Hamcrest matchers to Expecta in the object mapper tests to fix infinite recursion on test failure (will need to completely phase out Hamcrest soon)
* Provides for the registration of route either by class and method or via symbolic name
* Changed routes representation from simple dictionary to RKRoute model
* Eliminated RKRouter as a protocol
* Migrated RKObjectRouter to Network layer and renamed to RKRouter
* Added router as property of RKClient, RKObjectManager now proxies access
