Fixed destructive buttons causing an assertion to fail on alert views
on iOS 8.
Lots of changes, see description

- Added a compile-time error if used without ARC
- Renamed the `onTapped` parameter to `buttonAction` (and `tappedBlock`
to `buttonBlock`)
- Removed `show` and `showInView:` from the interface to enforce use of
the universal methods
- Made all `NSString` properties `copy`, and removed `assign` from
`destructiveButtonIndex` and `style`
- `destructiveButtonIndex` now defaults to `NSNotFound` instead of `-1`
- Corrected the return type of the initializers to `instancetype`
- Removed `didPressButtonAtIndex:` from the header by making it a
private protocol