JSON API: Included - consider nested associations

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

Start by reading the JSON:API include specification linked in the issue, then locate Grails' JSON API include-handling entry point. Trace how a direct association is currently processed and determine where one-level nested paths such as comments.author would be handled. Done means nested associations up to one level are supported and covered by appropriate tests.

Written by the indexing model from the issue text.