Serialize recursively placing all nested objects in the 'included' array

[rhyek]

Added functionality that recursively serializes a model, placing all nested objects in the included array. This conforms to the JSON API spec, which states:

http://jsonapi.org/format/#document-top-level

A document MAY contain any of these top-level members:

• jsonapi: an object describing the server’s implementation
• links: a links object related to the primary data.
• included: an array of resource objects that are related to the primary data and/or each other (“included resources”).

[joukevandermaas]

Thanks a lot for doing this! I will review your changes this weekend and hopefully publish a new pre-release.