Use resource-based stubs which are automatically namespaced

Was working on the docs for the `ember generate api-stub` command and though it could be nice to actually do this resource-based instead of route-based. This is a first version but it sould basically make the code out-of-the box compatible with ember-data..

An alternative would be to use `ember generate api-stub --resource <resource_name>` to override the current behavior.