method load_resource for controllers

This elegant solution follows rails 4 restful controller tendencies,
makes nested resources easier to understand
and leaves room for one's preffered authorization gem or custom logic.

![load_resource_rails](https://f.cloud.github.com/assets/335238/447346/db2a2bce-b1f6-11e2-9622-86906c686765.png)

Thank you guys for the hard work on helping developers.