Quantizedmesh vertex normals

Going public with current implementation of quantized-mesh per vertex normals.  This is a work in progress; will be improving doc, adding unit tests, and reverting changes to CesiumViewer.
There's not really any difference between the two comparisons, right?  data.extensions is a json list of strings.  I'm just testing to see if the string 'vertexnormals' is in that extensions list, meaning that it would have an array index of 0 or greater.