New materials

This is from a branch I started a few months ago.

* Removed `erosion` property from `Polygon`, `ComplexConicSensorVolume`, `RectangularPyramidSensorVolume`, and `ComplexConicSensorVolume`.
* Added new `RimLighting` and `Erosion` materials.
* Updated the [Fabric](https://github.com/AnalyticalGraphicsInc/cesium/wiki/Fabric) wiki page.