Mass clear action implemented

Adds a clearCompleted action to the todos controller and wires this to
the "clear completed" button.
Implement check/uncheck all button

This is done with a two-way computed property that allows either
returning whether every item is checked or setting the done state of
every item.
Add local storage adapter to dependencies

Done via npm. Once installed, the pipeline picks this up automatically.

    $ npm install --save-dev ember-localstorage-adapter
    ember-localstorage-adapter@0.5.0 node_modules/ember-localstorage-adapter
Swap application adapter

Removes the fixture adapter and replaces it with the local storage
adapter instead.