Add [UIAlertView rac_willDismissSignal].

I needed this in my project since `rac_buttonClickedSignal` does not fire when an `UIAlertView` is dismissed programmatically via `[UIAlertView dismissWithClickedButtonIndex:animated:]`.

The same change would likely make sense for `UIActionSheet`.

Can you please use tabs instead of spaces, and format this similarly to the other method?