Identity attribute related swift

[ManueGE]

If your JSON has the relationships by referencing the identity attribute instead of by nesting JSONs, you can take advantage of the identityAttributeRelated attribute. For example, if your JSON Dictionary is:

let batmanJSON: JSONObject = [
    "name": "Batman",
    "id": "1699",
    "publisher": 1 
]

you may not want Groot to serialize this dictionary by setting 1 to the Publisher relationship, but by assigning it with the Publisher object which has 1 as its identityAttribute value. You accomplish that by associating the identityAttribute key with true (or any other positive boolean value) in the entity user info dictionary:

This way, Groot will search in the publisher entity for a entry with the given identity attribute. If it is found, it will assign this entry as the Publisher object of the Character; if it doesn't Groot will create a placeholder Publisher entry with this value for the identity attribute. This placeholder object would be filled eventually if the new data is provided.

Note: To make this feature works, you must always to use the merge methods instead of the insert ones.

---

Include test cases

[gonzalezreal]

Closing as this has been superseded by #26