Next release

- [x] Support Asynchronous Update
- [x] Don't use `window-configuration-change-hook`
- [x] Fix by @m2ym suggestions
- [x] Fix by Stefan Monnier suggestions
- [x] Drop Emacs 23 support
- [x] Update documentation

- [x] Check with Emacs 24.1
- [x] Check git-gutter-fringe