First draft implementation of pervertex normals for quantized-mesh.  Will need to update other meshes that use the CentralBody shaders.
Added new request headers to that specifies the content type as quantized-mesh or a quantized-mesh with vertex normals.  Added flags to CesiumTerrainProvider that specify (1) should the client request vertex normals and (2) does the server have vertex normals.
Minor cleanup.
If no vertex normals are in the mesh, then do not add junk normal data to the vertex buffer.  Fixed Globe shader so that day-night shading can be used again.
Updated hasWaterMask and hasVertexNormals to be properties.
Added unit test to load QuantizedMeshTerrainData with VertexNormals.