PoC for serializing HAL payloads as vanilla JSON

- Pull "payload" and/or "documentation" key, and use as the variables to
  serialize.
- If variables are HAL, pull entity or collection.
Finalize HAL marshaling in JsonModel

- Do not fetch "documentation" top-level key;
  ZF\Apigility\Documentation\Controller actually sets both "documentation" and
  "type" keys, and, for top-level, only "apis".
- Moved Hal\Entity/Hal\Collection checks outside of "payload" conditional, in
  order to test for them at the top-level as well.
- Use ->getCollection(), not ->collection.
- Fully unit-tested.
Merge branch 'feature/serialize-hal'

Fixes an issue reported by @harikt on the mailing list, whereby when Json is
selected as the content-negotiation selector of a REST service, the payload was
not serialized at all. This patch checks for HAL payloads, and, if present,
pulls the raw entity or collection and serializes that.