Day-night lighting for models

Models are lit based on the sun direction but there's no recognition of a day-night cycle. Two of the problems are:

* The sun basically shines through the ground during night, lighting the bottom-facing part of the model
* The ambient lighting doesn't adjust to time of day.

Should there be a day-night lighting option on Models/3D Tiles?

Requested on the forum: https://groups.google.com/forum/#!topic/cesium-dev/8Y2lHMgJZVM.

Start by reviewing the Models and 3D Tiles lighting behavior described in the issue, along with the linked forum request. Determine how sun direction and ambient lighting currently respond to time of day, then define what a day-night lighting option should cover, including preventing sunlight from lighting bottom-facing surfaces at night. Done means the scope and expected lighting behavior are specified and implemented for the relevant model types.

Written by the indexing model from the issue text.