Use Ember.Evented for redirect, success and error events

Instead of directly defining the `onRedirect`, `onSuccess` and `onError` callbacks directly on the object, this pull request extends the `Ember.OAuth2` object with `Ember.Evented` so that one can define more callbacks, and be more specific.

`Ember.Evented` has support for `on`, `one` and `off`, this library can now use this.

It should maintain backward compatibility with the previous syntax, but will log deprecation notices to the console when detected.