Added event listener to the completed checkbox, which triggers the ToDo Model's toggle() method.  We also listen for the status of the model's isComplete property anytime the view is rendered, to add/remove the completed class from the <li> item.
Added placeholder function for listening to the custom 'visible' event on the model.  Eventually this will tie in with the Router to determine if the ToDo should be shown or not.
Added the Destroy method to the individual ToDo View and Model to remove items when the X is clicked.