[Story #11961455] Implements support for sending nested objects back to the remote server for processing:

* Introduce RKObjectMappable#relationshipsToSerialize to define nested relationships to post
* New helper methods for working with relationship serializations.
* Extended Rails router to serialize nested objects according to Rails idioms

Code/ObjectMapping/RKObject.m

@@ -15,6 +15,10 @@
 	return nil;
 }
 
++ (NSArray*)relationshipsToSerialize {
+	return [NSArray array];
+}
+
 + (NSDictionary*)elementToRelationshipMappings {
 	return [NSDictionary dictionary];
 }
@@ -27,4 +31,8 @@
 	return RKObjectMappableGetPropertiesByElement(self);
 }
 
+- (NSDictionary*)relationshipsForSerialization {
+	return RKObjectMappableGetRelationshipsByElement(self);
+}
+
 @end