Added support for loading a watermask for Quantized-Mesh terrain tiles.

These changes add support for the new quantized-mesh extension, watermask.  This extension is meant to be a drop-in replacement for the heightmap-1.0 terrain watermask, and shares the same format; 1 byte if all land/water, 64k mask for tiles with a mix of land and water.
Yes.  There will be a PR to the cesium-website today to update the spec document.  The format defined in heightmap-1.0 (http://cesiumjs.org/data-and-assets/terrain/formats/heightmap-1.0.html) was adopted for this extension.