Uses Ruby Observable module.

This will allow client applications to define their own observers.

Next steps:
* Define Merit accessor for the observable (`Judge`).Something like:
  `Merit.observable.add_observer MyAppsObserver`
* Define best API for clients' Observers.
* Add an App observer to `test/dummy/` Rails app.

Pairing with @marcinsklodowski
Added observer to Merit.

By default LogReputationChange is added to Merit.observers

Also Merit.observers is just an array, so user can add Observer likt that:
Merit.observers << CustomObserverClass

judge object gets observers from Merit.observers on initialization and notifies observer objects if changed.