Rewrote the #observe method to instantiate an Observer

This allows things like StateMachine, which integrate with AR by extending the ActiveRecord::Observer class, to function correctly with the Dependency.
E.g. you can now do:

  live_resource ... do |resource|
    # Where :after_parked and :before_idling are StateMachine events
    depends_on(Car, :after_parked, :before_idling) do |car, event|
      ...
    end
  end