Fixed  delegate method forwarding issue.
Implement respondsToSelector:.
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.
Chagne DemoApp to new interface
Merge pull request #3 from ninjinkun/fix/delegate-method-not-forward

Fixed delegate methods are not forwarded