User コントローラの実装

Sign up, Sign in 機能の実装
----------

## Sign up
* GET /users/sign_up
* description
    * This api create user
* parameter
    * email: string
    * password: string
    * confirm_pw: string
* return
    * true / false

## Sign in
* GET /users/sign_in
* description
    * This api authenticate user
* parameter
    * email: string
    * pasword: string
*  return
    * id: int

## Sign out
* DELETE /users/sign_out
* description
    * This api sign out user
* parameter
    * id: int
*  return
    * true / false