JSON API: Included - consider nested associations

[ColinHarrington]

Add support for an entities nested associations up to one level like an Aritcles' comments.author in the following example: http://jsonapi.org/format/#fetching-includes

[majkelo]

We could also extend such a nested associations in gsons as mentioned in thread http://stackoverflow.com/questions/40441814/second-level-expand-in-gson