Fixed delegate methods are not forwarded

# Problem
UITableViewDelegate methods are not called. https://github.com/ninjinkun/NJKScrollFullScreen/issues/2

# Solution
- Remove scrollViewDelegate property.
- Introduce new initializer `initWithForwardTarget:`.

Delegate caller (e.g. UITableView) checks target implemented delegate methods with respondsToSelector: and caches these results when call setDelegate:. Therefore, forward target needed before set NJKScrollFullScreen to scroll view delegate. initWithForwardTarget: registers forward target at initialize timig.

```objc
- (void)viewDidLoad
{
    [super viewDidLoad];

    _scrollProxy = [[NJKScrollFullScreen alloc] initWithForwardTarget:self]; // UIScrollViewDelegate and UITableViewDelegate methods proxy to ViewController
    self.tableView.delegate = (id)_scrollProxy; // cast for surpress incompatible warnings
    _scrollProxy.delegate = self;
}
```