Adds support for UIAlertController

Exposes private `-handler` to `UIAlertAction` in spec. Adds `-dismissByTappingCancelButton` and -`dismissByTappingButtonWithTitle:` to `UIAlertController`.

The API follows a similar pattern to PCK's existing `UIActionSheet` but renames to be closer to Apple's guidelines. We deduced the behavior of `UIAlertController` two styles by creating an example project and experimenting with different combinations of button styles.

Note that the class method, `[UIAlertView currentAlertView]` and `[UIActionSheet currentActionSheet]`, will not return a `UIAlertController`, use `UIViewController`'s `-alertController`.