Add Support for UIAlertView alertViewShouldEnableFirstOtherButton: delegate method

I had the need to use the alertViewShouldEnableFirstOtherButton: delegate method of UIAlertView in an internal project, and I thought it would be nice to include in this library.  There is an interesting issue (bug?) with UIAlertView where the alertViewShouldEnableFirstOtherButton delegate method won't be called if you don't init the UIAlertView with at least one otherButtonTitle.  Adding an otherButton post-init via addButtonWithTitle: doesn't seem to change the behavior.  This PR includes a change to the base UIAlertView that should fix that issue.