Add models from relations

/to @bajtos 
/cc @STRML

The PR adds related models into the Swagger spec so that REST operations for relations are correctly typed.
The trouble is, the reader looking at the following code has no idea where are `testModel` and `relatedModel` coming from.

```js
      var defs = getDefinitions({
        str: String // 'string'
      });
      expect(defs).has.property('testModel');
      expect(defs).has.property('relatedModel');
```

I suppose you can rename `getDefinitions` to `getDefinitionsOfTestModelWithRelatedModel`, but then the code in my comment above may be easier to comprehend.