Bind change event to Items and reflect changes in its model

* Changed .render() to .renderRaw() which returns only an HTML string
* Add $.trim() to .renderRaw() to make it possible to transform the string return into an jQuery element
* Update .render() to return a jQuery element with events bound to it
Make sure items work independently
Bind destroy event to .destroy button

* Add new class to Item checkbox to improve selectors and update tests
* Clicking on button.destroy removes its element from the DOM