如下，一个UIContainerView内切换两个Controller，当点击登录的时候UIContainerView的视图为LoginController，当点击登记的时候UIContainerView中的视图为RegisterController
在主视图内先做两个按钮的事件。这个不用说啦。当触发按钮事件的时候调用以下代码就可以。
var newController = self.storyboard?.instantiateViewControllerWithIdentifier("RegisterController") as! RegisterController
let oldController = childViewControllers.last as! UIViewController
oldController.willMoveToParentViewController(nil)
addChildViewController(newController)
newController.view.frame = oldController.view.frame
//isAnimating = true
transitionFromViewController(oldController, toViewController: newController, duration: 0.1, options: UIViewAnimationOptions.TransitionNone, animations: nil, completion: { (finished) -> Void in
oldController.removeFromParentViewController()
newController.didMoveToParentViewController(self)
//self.isAnimating = false
})
