Connect related objects when using relationship-type RKRoute

This proposed addition allows users to automatically connect child objects to their parent object when using a relationship-style `RKRoute`. It is meant to complement `RKConnectionDescription`. See the following condensed fictional example.

Entities are as such:

```objective-c
@interface Photo : NSManagedObject
@property (nonatomic, retain) NSNumber * photoID;
@property (nonatomic, retain) NSURL    * URL;
@property (nonatomic, retain) User     * author;
@end

@interface User : NSManagedObject
@property (nonatomic, retain) NSNumber * userID;
@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) NSSet    * photos;
@end

@interface User (CoreDataGeneratedAccessors)
- (void)addPhotosObject:(Photo *)value;
- (void)removePhotosObject:(Photo *)value;
- (void)addPhotos:(NSSet *)values;
- (void)removePhotos:(NSSet *)values;
@end
```

RestKit is configured as such:

```objective-c
RKObjectManager *manager = [RKObjectManager managerWithBaseURL:[NSURL URLWithString:@"http://restkit.org"]];

// configure CoreData here (truncated)

RKRoute *userPhotos = [RKRoute routeWithRelationshipName:@"photos" objectClass:[User class] pathPattern:@"/user/:userID/photos" method:RKRequestMethodGET];
userPhotos.connectRelatedObjects = YES; // here's where the magic happens
[manager.router.routeSet addRoute:userPhotos];

// this is all standard and straightforward - included merely for completeness
RKEntityMapping *photoMapping = [RKEntityMapping mappingForEntityWithName:@"Photo" inManagedObjectStore:manager.managedObjectStore];
photoMapping.identificationAttributes = @[@"photoID"];
[photoMapping addAttributeMappingsFromArray:@[@"photoID", @"URL"]];

RKResponseDescriptor *photosResponse = [RKResponseDescriptor responseDescriptorWithMapping:photoMapping pathPattern:@"/user/:userID/photos" keyPath:nil statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassSuccessful)];
[manager addResponseDescriptor:photosResponse];
```

The request is performed as such:

```objective-c
User *user = self.user; // assume user exists
[[RKObjectManager sharedManager] getObjectsAtPathForRelationship:@"photos" ofObject:user parameters:nil success:nil failure:nil];
```

If the request is successful, all of the returned photos will be assigned to the photos set of the user object which was passed as the parameter of the ```getObjectsAtPathForRelationship:``` call. Otherwise everything continues as normal.

Known problems with this suggested patch:

* It introduces CoreData baggage into the ```RKRoute``` class.
* It does not attempt to delete objects which have been removed on the server.

I am open to discussion. I am using this patch in my own code and thought others might find it useful.

Thanks,

Andrew