Low-level changes for models

A subset of the [models branch](https://github.com/AnalyticalGraphicsInc/cesium/tree/models):

* Added `fromUniformScale` to `Matrix2`, `Matrix3`, and `Matrix4`.
* Added `fromScale` to `Matrix2`.
* Added `multiplyByUniformScale` to `Matrix4`.
* Added `flipY` property when calling `Context.createTexture2D` and `Context.createCubeMap`.