Added KVO for UIScrollView contentOffset

If you add a noticeView to a scrollView you can use KVO to detect user's scroll and move the view.

Example:
notice = [WBSuccessNoticeView successNoticeInView:self.view title:@"Test"];
[notice setOriginY:self.tableView.contentOffset.y];
__weak WBNoticeView *noticePointer = notice;
[self.tableView addObserver:notice forKeyPath:@"contentOffset" options:NSKeyValueObservingOptionNew context:NULL];
[notice setDismissalBlock:^(BOOL dismissedInteractively) {
[self.tableView removeObserver:noticePointer forKeyPath:@"contentOffset"];
notice.view = nil;
}];
[notice show];