Pluralize generated api-stub routes

This commit makes the api-stub generated server routes have
the same path as Ember model generated routes. For example,

    `ember g api-stub person`

... will generate a stubbed route at /api/people. This
behaviour is matched on the Ember side, with this

    `ember g resource person`

... generating the path /people/:person_id for the
Ember route.
ping?