Add property for checking if all todos are done

This property will be true if the controller has any todos and every
todo's isCompleted property is true. If the isCompleted property of any
todo changes, this property will be recomputed. If the return value has
changed, sections of the template that need to update will be
automatically updated for us.
Toggle complete/incomplete on all todos

If no value argument is passed this property is being used to populate
the current value of the checkbox. If a value is passed it indicates the
checkbox was used by a user and we should set the isCompleted property
of each todo to this new value.

The count of remaining todos and completed todos used elsewhere in the template automatically re-render for us if necessary.
Add localstorage adapter and use instead of fixture

This uses 'ember-localstorage-adapter' which will work with ember-data
for saving application information in localStorage