Add functionality to edit and re-render particular view

* initialize listens for any changes and re-renders the todo
* render replaces and appends the html of the single todo
  template. also caches this.$input as the title
* events hash has three methods:
  * edit: change to edit mode when double click a todo
  * updateOnEnter: closes edit mode when hit enter
  * close: trims the input. if input exists, we save the
    todo with its new title and get out of edit mode