/**
  * Get a user by _id
  */
 * get (userId) {
   let user = yield UserModel.findById(userId, Studio.defer())
   if (!user) {
     throw (new Error('No user found by that _id.'))
   } else {
     return user
   }
 }
 /**
  * Create a new user
  */
 * create (userData) {
   let newUser = yield new UserModel(userData).save(Studio.defer())
   return Services.Auth.sign(newUser._id)
 }