Implement Animations

```swift
func add(CAAnimation, forKey: String?)
Add the specified animation object to the layer’s render tree.

func animation(forKey: String) -> CAAnimation?
Returns the animation object with the specified identifier.

func removeAllAnimations()
Remove all animations attached to the layer.

func removeAnimation(forKey: String)
Remove the animation object with the specified key.

func animationKeys() -> [String]?
Returns an array of strings that identify the animations currently attached to the layer.
```