Added frustum offset to PerspectiveFrustum.

Added a simple frustum offset property to PerspectiveFrustum. This is something we require for the [Cesium Oculus/VR plugin](https://github.com/NICTA/cesium-oculus-plugin/tree/webVR) and could be useful for others.
I could separate this into individual `x` and `y` setters and set them up using the `defineProperties`?