Improved templates root path matching

With this change we can have nicely isolated ember applications inside a single rails app.
For example, in your assets/javascripts you could have:

- frontend.js
- admin.js
- frontend/templates/application.handlebars
- admin/templates/application.handlebars

And in each application, its template would be available at `application` path, instead of `[app_name]/templates/application` (assuming that config.handlebars.templates_root == 'templates').