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.
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
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
Merge pull request #128 from marcinsklodowski/observer-the-real-one

Added observer to Merit.