Use a property to store RKObjectRequestOperation on RKHTTPRequestOperation

This allows consumers of RestKit to pay attention to NSNotifications sent by AFNetworking and find out which RKObjectRequestOperation they correspond to.

For instance, I have a table view whose data source is backed by a collection of data objects that are fetched from the server via RestKit. In order to simplify table view cell insertions/deletions/reloads, I use KVO. The `observeValueForKeyPath:ofObjectchange:context:` implementation calls the appropriate `insertRowsAtIndexPaths:withRowAnimation:`/`deleteRowsAtIndexPAths:withRowAnimation:` invocations as the values of properties are changed by RestKit's mapping engine.

In order to have consistent animations, I make all of these invocations inside a `startUpdates`/`endUpdates` pair. This means that `startUpdates` needs to be called before RestKit performs any mapping, and `endUpdates` is called after RestKit completes all mapping. Calling `endUpdates` is easy since I can do it from the `success`/`failure` blocks passed into `putObject:path:parameters:success:failure:`. Calling `startUpdates` at the correct time requires listening to `AFNetworkingOperationDidFinishNotification` since it's sent before RestKit performs mapping.

The `object` attached to this `NSNotification` is an `RKHTTPRequestOperation`, but in order to know how to treat the notification, the code consuming this notification needs to know what the owning `RKObjectRequestOperation` instance is, in order to check whether the `targetObject` matches the expected object.

Some sample code (modified and simplified - in my real code, I'm not passing a `UITableView` into a model method :wink:)

```objc
// MyBaseModelClass.m

- (void)putObjectAffectingTableView:(UITableView)tableView {
    __weak THRModel* weakSelf = self;
    __block id observer = nil;

    // We want to call [tableView beginUpdates] right before RestKit does its mapping
    observer = [[NSNotificationCenter defaultCenter] addObserverForName:AFNetworkingOperationDidFinishNotification object:nil queue:nil usingBlock:^(NSNotification *note) {
        if (weakSelf == nil) {
            [[NSNotificationCenter defaultCenter] removeObserver:observer];
            return;
        }
        
        if ([note.object isKindOfClass:RKHTTPRequestOperation.class]) {
            RKHTTPRequestOperation* http = (RKHTTPRequestOperation*)note.object;
            RKObjectRequestOperation* op = http.objectRequestOperation;

            // The request that this notification is about ... 
            // is it the one that we were sending when we created this observer?
            if (op && op.targetObject == weakSelf) {
                [tableView beginUpdates]
                [[NSNotificationCenter defaultCenter] removeObserver:observer];
            }
        }
    }];

    [objectManager putObject:self path:[self.url stringByAppendingString:@".json"]
                  parameters:nil
                     success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult) {
                         [tableView endUpdates]
                     }
                     failure:^(RKObjectRequestOperation *operation, NSError *error) {
                         [tableView endUpdates]
                     }];
}
```