Support spatial lookup of assets

Assets should not be provided in its actual responses to Sockets - potentially it might be supplying the whole massive scene if the viewpoint was far enough back and localities were wide enough. Instead, it should provide references to assets, for the client to pull asynchronously.

Bad Idea

{
    /* Target node SID path:
     */
    "#root/alpha": {
 
        /* Maps to node's addNode method:
         */
        "+node" :
                SceneJS.node({ sid: "asset789" }, // Note the asset ID
                        SceneJS.rotate({ angle: 45, y: 1.0 },
                                SceneJS.translate({ x: 100 },
                                        SceneJS.objects.teapot())))
    }
}

Better Idea

SceneJS.kdAsset is an extension node for asset server kd-tree client scenes, which will be defined alongside the client Socket subtree within a SceneJS content module. It will asynchronously pull the asset from the server using JSONP.

{
    /* Target node SID path:
     */
    "#root/alpha": {
 
        /* Maps to node's addNode method:
         */
        "+node" : SceneJS.kdAsset({ uri: "http://assets.scenejs.org?cmd=getAsset&id=org.scenejs.models.myTeapot" })
    }
}