Make Ember.observer work with the function as the last argument. Support the old style with a deprecation notice.
Update tests with the new style for Ember.observer.
Update Ember.immediateObserver and Ember.beforeObserver to match the new Ember.observer style.
Only soft-deprecate the old Ember.observer style.
User Ember.Error for errors.
Merge pull request #3590 from gunn/master

Change Ember.observer and friends to take function as the final argument