Allow to associate any type of object

[Igor-Palaguta]

Very useful to associate any type of object: structs/enums
currently associatedObject allows to associate just AnyObject instances

private class Box<T> {
   let unbox: T
   init(_ value: T) {
      self.unbox = value
   }
}

extension NSObject {
   public final func associatedObject<T>(key key: UnsafePointer<()>, @noescape initial: () -> T) -> T {
      if let boxedObject = objc_getAssociatedObject(self, key) as? Box<T> {
         return boxedObject.unbox
      }
      let object = initial()
      objc_setAssociatedObject(self, key, Box(object), .OBJC_ASSOCIATION_RETAIN)
      return object
   }
}