Properly initialises the class when using UIAlertView’s designated initialiser

Fixes #8 — the UIAlertView designated initialiser expects the `otherButtonTitles` parameter to be non-nil before the `- (BOOL)alertViewShouldEnableFirstOtherButton:(UIAlertView *)alertView` delegate method will be called.

I've also filed a radar: http://openradar.appspot.com/radar?id=6189750637559808