Introduce edit-todo component

This will be made available via Handlebars for editing task items.

    $ ember generate component edit-todo
    version: 0.1.2
    installing
      create app/components/edit-todo.js
      create app/templates/components/edit-todo.hbs
    installing
      create tests/unit/components/edit-todo-test.js
Base new edit-todo helper on Ember.TextField

This adds an automatic focus to the base TextField class so that it gets
focused immediately after being inserted into the DOM.