SDCAlertView buttons deselected once tapped.

I noticed that buttons are not deselected once they have been tapped. Ordinarily this is not a problem as the alert is dismissed as soon as a button is tapped. But if the alert view continues to be shown after a button has been pressed  then the problem is noticeable.

Replicate the problem by setting a `shouldDismissHandler` of the form:

```
[alert setShouldDismissHandler:^BOOL(NSInteger buttonIndex) {
    return NO;
}];
```