[#44] Added test demonstrating new desired behavior

- Allow resources to return `ApiProblem` objects and short-circuit
  behavior
[#44] Allow returning `ApiProblem` from resources and RESTful methods

- Allows returning `ApiProblem` results from `Resource` listeners; these
  will be returned immediately by the various REST methods of the
  controller.
- This allows creating custom `ApiProblem` results that contain
  additional fields.
Merge branch 'feature/44-api-problem-returns' into develop

Fixes #44 -- allows resource listeners to return `ApiProblem` results, which are
in turn returned immediately by the controller.