Empty index template for todos

This will receive the list itself once setup.

    $ ember generate template todos/index
    version: 0.1.2
    installing
      create app/templates/todos/index.hbs
Relocate task list template

This moves it into the "index" for the resource, introducing an outlet
in the resource container.
Generate filtered routes

Add subroutes under the 'todos' resource.

    $ ember generate route todos/active
    version: 0.1.2
    installing
      create app/routes/todos/active.js
      create app/templates/todos/active.hbs
    installing
      create tests/unit/routes/todos/active-test.js

    $ ember generate route todos/completed
    version: 0.1.2
    installing
      create app/routes/todos/completed.js
      create app/templates/todos/completed.hbs
    installing
      create tests/unit/routes/todos/completed-test.js
Fix incorrect modification of router.js

The ember-cli tool does not correctly update the route by recognizing it
as a subroute of the 'todos' resource.
Wire up links in todos template

Replaces the static links with link-to helper ones.
Drop active/completed templates

Replaces these physical templates with an alias to the index template.
Configure model on the filter routes

Adds model definitions on the active and completed routes to allow only
certain todo items to be passed through to the controller.
Bind completed button visibility/count to model

This introduces a new "completed" calculated value and wires it to the
button.