Add route for Active todos

Replaces Active anchor with a link-to helper pointed at 'todos.active',
which links to #/active. The router was updated with the route based on
naming convention 'TodosActiveRoute', its model filters through the data
and returns todos whose 'isCompleted' property is false.

Normally transitioning into a new route changes the template rendered into
the parent {{outlet}}, but in this case we'd like to reuse the existing
todos/index template. We can accomplish this by implementing the
renderTemplate method and calling render ourselves with the specific
template and controller options.
Add route for displayed only completed todos

Similar to previous commit, except only show todos that have isCompleted
as true.
Update all links with handlebars link-to helper
Only display clear button if there are completed tasks

This adds a conditional to the view checking for the boolean value of
'hasCompleted', if true add a button to clear completed tasks along with
a count of the number of completed tasks.
Add action method to remove completed from model

The method is added to TodosController as an action and will filter the
model for todos that have 'isCompleted' as true, the returned todos will
be deleted and the model saved with the updates.
Add methods for removing completed

The completed and clearCompleted methods both invoke the filterProperty
method, which is part of the ArrayController API and returns an instance
of EmberArray which contains only the items for which the callback returns
true. The clearCompleted method also invokes the invoke method which is
part of the EmberArray API. invoke will execute a method on each object
in the Array if the method exists on that object.
Update input with handlebars helper

This checkbox will be checked when the controller property 'allAreDone'
is true and unchecked when the property 'allAreDone' is false.