Introduce the id type equivalent

[gabrielf]

As explained here: http://stackoverflow.com/questions/24005678/what-is-the-equivalent-of-an-objective-c-id-in-swift

[nickoneill]

Hi @gabrielf, while I agree that id was the accepted way to do this in Objc and AnyObject is a good direct translation into Swift, the point of these posts is to discover the "right" way to do this in Swift.

In this case, a cast in the method signature makes more sense to me here than just using id and then requiring a line of let sender = sender as UIButton in the function before you use it.