Promote '201 Created' HTTP status

The HTTP spec includes the '201 Created' HTTP status and its
accompanying Location header specifically for situation such as
described in the "Creating" section of jsonapi.org.

* Retains the existing '204 No Content' and '200 OK' responses for
  compatibility with existing frameworks, but adds the '201' response
  and promote it as the preferred response.

* Restricts the availability of the '204 No Content' response to
  situations where the client can accurately determine the server
  location of the newly created resource.

* Fixes some 'posts' vs. 'photos' typos in the examples.