Change SceneJS.Geometry "type" to "resource"

Required for: #162 Remove "cfg" from node JSON configuations

To avoid JSON clash with node "type", Geometry "type" becomes "method". This optional property identifies its VBOs as being shared with other Geometries that specify the same "resource" value.

Eg:

{
   type: "geometry",
   resource: "myBox",
   //...
}