Added support for polymorphic object mapping (Github #105, #244). This enables you to dynamically map objects to different destination classes or using different mapping strategies via configuration or callbacks. See Docs/Object Mapping.md for details.

Other changes include:
* Eliminated the RKObjectFactory protocol and implementations. Object mapping instances themselves are
now responsible for instantiating target objects for mapping.
* Introduced RKObjectAbstractMapping superclass for RKObjectMapping and RKObjectPolymorphicMapping.
* Updated example applications to use block object loaders (RKTwitter and RKTwitterCoreData)
* Refactored method signatures of RKObjectMapper, RKObjectMapping, and RKObjectMappingProvider to reflect the
existence of abstract mapping types. This was necessary to make polymorphic mappings integrate cleanly.
* Fixed overlap in RestKit error domains between network and object mapping. fixes #208

Code/ObjectMapping/RKObjectMapperError.h

@@ -9,8 +9,9 @@
 #import "../Support/Errors.h"
 
 typedef enum RKObjectMapperErrors {
-    RKObjectMapperErrorObjectMappingNotFound,       // No mapping found
-    RKObjectMapperErrorObjectMappingTypeMismatch,   // Target class and object mapping are in disagreement
-    RKObjectMapperErrorUnmappableContent,           // No mappable attributes or relationships were found
-    RKObjectMapperErrorFromMappingResult
+    RKObjectMapperErrorObjectMappingNotFound        = 1001,     // No mapping found
+    RKObjectMapperErrorObjectMappingTypeMismatch    = 1002,     // Target class and object mapping are in disagreement
+    RKObjectMapperErrorUnmappableContent            = 1003,     // No mappable attributes or relationships were found
+    RKObjectMapperErrorFromMappingResult            = 1004,     // The error was returned from the mapping result
+    RKObjectMapperErrorValidationFailure            = 1005      // Generic error code for use when constructing validation errors
 } RKObjectMapperErrorCode;