Return user id for created user

When sending a POST request to create a `User` there was no way
to know the URL to the new user or even its ID. This adds a
Location header to be more RESTful to the response as well as
returns the id in the response body.

This changes how the `UserService::create()` is used because it
requires changing from returning true/false to returning the `User`
object which was saved or `null`.

Resolves #69