Added ApiProblemResponse and response sender

- ApiProblemResponse is capable of "rendering" itself.
- SendApiProblemResponseListener ensures that stack traces are included
  when display_exceptions is enabled
Modify dispatch.error listener

- Now returns an ApiProblemResponse (and sets it in the MvcEvent),
  instead of an ApiProblemModel.
Merge pull request #3 from zfcampus/feature/api-problem-response

Added ApiProblemResponse and response sender