Timout on SceneJS.Instance nodes

Since these may instance Symbol nodes defined within asynchronously-loaded content, they should tolerate the absence of Symbols, however should also raise an exception when Symbols don't resolve after a timeout period.

An example in which this is required is below:

SceneJS.loadCollada({
    uri: "http://www.scenejs.org/library/v0.7/assets/" +
              "examples/seymourplane_triangulate/" +
                   "seymourplane_triangulate.dae"  ,
                           showBoundingBoxes: false
    }),

SceneJS.instance({
    name: "VisualSceneNode/camera1"       
}),