Add subidentifier (SID) to SceneJS.Node

Add an optional SID property to SceneJS.Node, which would get a String value that must be unique within the scope of it's parent SceneJS.Node.

var node = new SceneJS.Node({
         sid: "myNode"
       });

var sid = node.getSID();

The SceneJS.Selector is to allow selection of children by SID as well as index:

var selector = new SceneJS.Selector(
       new SceneJS.Node({ sid: "alpha" }),  
       new SceneJS.Node({ sid: "beta" }),  
       new SceneJS.Node());                   // No SID for third child

var selector.setSelection([ "alpha", 2 ]);    // Selects first and third nodes