Provide access to the associated ObjectMapper in JsonResource via default load/save options

[kchobantonov]

When combined with EMF mongo project we do need an access to the object mapper associated with the JsonResource so that we can use the predefined ObjectMapper in MongoInputStream/MongoOutputStream instead of creating new ObjectMapper via EMFModule.setupDefaultMapper();

Possible implementation

 ```
  public JsonResource(URI uri, ObjectMapper mapper) {
    super(uri);
    this.mapper = mapper;

    defaultLoadOptions = new HashMap<Object, Object>();
    defaultSaveOptions = new HashMap<Object, Object>();

    defaultLoadOptions.put(ObjectMapper.class.getName(), mapper);
    defaultSaveOptions.put(ObjectMapper.class.getName(), mapper);
}