各种代码应该写在什么位置，参考这个英文的：http://codefol.io/posts/Where-Do-I-Put-My-Code
文中提到一本电子书《Rebuilding Rails》，教你写框架，没人提过，有点意思。
Code in the application helper is included in all the views in your application. Code in other helpers is included in the corresponding view.
这段是错误的。
