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/RKObjectManager.h

@@ -281,6 +281,7 @@ typedef enum {
  POST a remote object instance and yield the object loader to the block before sending
  
  @see sendObject:method:delegate:block
+ - (RKObjectLoader*)postObject:(id<NSObject>)object delegate:(id<RKObjectLoaderDelegate>)delegate block:(void(^)(RKObjectLoader*))block;
  */
 - (RKObjectLoader*)postObject:(id<NSObject>)object delegate:(id<RKObjectLoaderDelegate>)delegate block:(void(^)(RKObjectLoader*))block;