Let subclasses access the dismissbutton.

We have a subclass, that can update the `messageLabel.text` while the user is typing. To be able to test that properly via KIF we also need to update the `accessibilityLabel` elements. Therefore I want subclasses to be able to access the `dismissButton` element as well as the `gradientView` element (that's there already).

![Screen Shot 2013-02-04 at 7 41 08 AM](https://f.cloud.github.com/assets/31597/122893/7f529682-6e9a-11e2-877d-185ea720f717.png)