Quantizedmesh vertex normals

Here is a gist that can be dropped into the sandcastle gallery for testing.
https://gist.github.com/abwood/e6b3ab977bc6b286f430
This and friends should have unit tests in [BuiltinFunctionsSpec.js](https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Specs/Renderer/BuiltinFunctionsSpec.js).