[SPA++] Always put HippoBean objects as direct children of the content section

When a document bean has a reference to another document bean, the referenced beans is always serialized as nested property object of the first bean by Jackson by default.
So, for example, if there's a reference chain like A -> B -> C documents, the content section in the json api output looks like the following:

{
  // SNIP
  "content": {
    "u001": {
      // content of A
      "refOfB": {
        // content of B
        "refOfC": {
          // content of C
        }
      }
    }
  }
}

See the following screenshot and example json output as-is:

international_atomic_time.html.json

I think it would much nicer and more intuitive if it should always look like this instead:

{
  // SNIP
  "content": {
    "u001": {
      // content of A
      "refOfB": {
        "$ref": "/content/u002"
      }
    },
    "u002": {
      // content of B
      "refOfC": {
        "$ref": "/content/u003"
      }
    },
    "u003": {
      // content of C
    }
  }
}

Not sure how it would be difficult to change the behavior with Jackson though yet...