Better implementation of UUAlert

I was the one who complaint on twitter about your code quality. I don't except you do pull my changes. But you wanted to know what I would change.

1. Never ever set a delegate without being able to reset it. It's a death trap.
2. Don't use static strings for texts 'OK', 'Ok' and 'Cancel' in this case.

It might be fine in your code. But if you publish code on GitHub people not as good as you will use the code and this might trip over this.