- The new style exports a function instead of the class. The
  exported function will take the auth context as a parameter
  and returns builder function.
- Files will be under a sub directory with versioned name,
  so that it can be exposed as a sub package of the API package.
- A few misc changes and style changes made to pass lint
  checker in gcloud.

- constructor parameters are not actually useful now. They are
  parameters to the builder function actually, so I moved there.
- build() is not a function, now it's a class. This way, JSDoc
  can generate documentations for its methods (like builder function).
- add an example clause in the constructor doc comments, so that
  documentation says the details of how to create an instance of
  the API client.