title: Instantiate a NSManagedObject without saving it
body: <p>I got a class like :</p>
  
  <pre><code>@objc(User)
  class User: NSManagedObject {
  
      @NSManaged var id: String
  
      //MARK: - Initialize
      convenience init(id: String,  context: NSManagedObjectContext?) {
  
          // Create the NSEntityDescription
          let entity = NSEntityDescription.entityForName("User", inManagedObjectContext: context!)
  
  
          // Super init the top init
          self.init(entity: entity!, insertIntoManagedObjectContext: context)
  
  
          // Init class variables
          self.id = id
  
      }
  }
  </code></pre>
  
  <p>I create a new User in a ViewController :</p>
  
  <pre><code>managedObjectContext = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext!
  
  var aUser = User(id: "500T", context: managedObjectContext)
  </code></pre>
  
  <p>I have another class "Group" where Group have many Users. To save user I do something like </p>
  
  <pre><code>aGroup!.users = NSSet(array: users!)
  !managedObjectContext.save
  </code></pre>
  
  <p>I don't want to save all users. How can I instantiate a user without saving it ?</p>