async function login(req, res, next) {
  res.status(HTTPStatus.OK).json(req.user.toAuthJSON());

  return next();
}
@api {post} /users/login Login a user
@apiDescription Login a user
@apiName loginUser
@apiGroup User

@apiParam (Body) {String} email User email.
@apiParam (Body) {String} password User password.

@apiSuccess {Number} status Status of the Request.
@apiSuccess {String} _id User _id.
@apiSuccess {String} token Authentication token.

@apiSuccessExample Success-Response:

HTTP/1.1 200 OK

{
 _id: '123',
 token: 'JWT eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJfaWQiOiI1OTBhMWI3ODAzMDI3N2NiNjQxM2JhZGUiLCJpYXQiOjE0OTM4MzQ2MTZ9.RSlMF6RRwAALZQRdfKrOZWnuHBk-mQNnRcCLJsc8zio',
}

@apiErrorExample {json} Error
 HTTP/1.1 400 Bad Request

 {
   email: 'email is required'
 }