Complete and delete todos

  * Click on complete:
    * invokes toggleComplete, which calls toggle on the todo
    * toggle on them model changes the completed status and saves
    * save generates a change, so the view is updated
    * all triggers change:completed,which is handled by the filterOne
      filterOne triggers a visible event on the model
    * the visible event invokes toggleVisible, which uses isHidden
      to determine if a todo should be displayed
  * Click on delete
    * clear method called, which calls destroy on the model
    * todo is deleted, and the destroy event is triggered
    * deletes the view, as we bound a listener for model destroy
    * model is removed from collection, triggering remove event
    * appview is listening for all events, so everything is updated