register - add agent method

// ...

const Auth = {
  // get current user's user object
  current: () => requests.get('/user'),
  // get promise for login success/failure
  login: (email, password) =>
    requests.post('/users/login', {user: {email, password}}),
  register: (username, email, password) =>
    requests.post('/users', { user: { username, email, password } })
};

// ...