WIP: Observes-hash mixin

- [x] `mergeFromSuper` / `concatFromSuper`
- [x] support array of handlers for the same event
- [ ] port `WBViewPresenter#createObserveBindings` to use this mixin
- [ ] WBViewPresenter like bindings with `*` & `>`
- [ ] support for `once` bindings
- [ ] specs