Support UIBarButtonItem with UIButton customView

In my application, I need to set up `rac_command` for UIBarButtonItem

```objC
    UIImage *backImage = [UIImage imageNamed:@"back-arrow-button.png"];
    UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
    [button setImage:backImage forState:UIControlStateNormal];
    self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:button];

    self.navigationItem.leftBarButtonItem.rac_command = [[RACCommand alloc] initWithSignalBlock:^RACSignal *(id input) {
        return [RACSignal createSignal:^RACDisposable *(id<RACSubscriber> subscriber) {
            // do some thing in here
            return nil;
        }];
    }];
```

But since UIBarButtonItem with customView does not support set `target` and `action`, so I need to make a new variable of class to store `self.button`

This fix will resolve this issue