Added support for object groups in the SceneLoader. "groups" is a new property in the SceneLoader result object.

Ex:
input {
  //...
  "objects" : {
    "Object.1" : {
      "groups"    : [ "Group.A" ],
      //...
    },
    "Object.2" : {
      "groups"    : [ "Group.A", "Group.B" ],
      //...
    },
    "Object.3" : {
      "groups"    : [ "Group.B" ],
      //...
    }
  }
}

result {
  //...
  groups: {
    'Group.A': [ 'Object.1', 'Object.2' ],
    'Group.B': [ 'Object.2', 'Object.3' ]
  },
  //...
}