Added ApiProblemResponse and response sender

- ApiProblemResponse is capable of "rendering" itself.
- SendApiProblemResponseListener ensures that stack traces are included
  when display_exceptions is enabled

Essentially, this provides a way for listeners to return an ApiProblem "early", largely removing the necessity of a dedicated renderer. This may also answer some critiques from @ocramius regarding the usage of ApiProblem and ApiProblemModel as return values.