UIScrollView 與 Auto Layout

讓 UIScrollVIew 自己管理 content size

• 用一個 UIView 當做 container (contnetView)
• container 與 self.view equal width

- UIView
    - UIScrollView
        - UIView (contentView) <--- 與 self.view equal width
            - UIView (topContainer)
            - UIView (bodyContainer)
            - UIView (footerContainer)
            ...etc