CZML Model support.

Models still need to be implemented on the client side, but the CZML should be straight forward.  Here's an example of what this pull allows.

```
{
  "id" : "Vehicle",
   "position" : {
    "cartographicDegrees":[0.0,0.0,0.0]
  },
   "orientation" : {
    "unitQuaternion":[0.0,0.0,0.0,1.0]
  },
  "model" : {
     "show" : true,
     "scale" : 1000.0,
    "uri" : {
       "webgltf" : "myModel.json"
       "dae" : "myModel.dae"
    }
}
```

So `model` depends on `position` and `orientation`.  You can specify `show`, `scale`, and `uri`.  The first two are straightforward.  The only special thing about `uri` is that it allows multiple formats to be specified for the same model file (based on property name).  The client then selects the model file that it has the "best" support for.  This is similar to how HTML5 video works and allows CZML to be platform agnostic.