Add destroy method to the Scene object

Other scene properties that should be set to ```null``` on destroy include any GL resources:

* ```this.fbo``` https://github.com/tangrams/tangram/blob/iwillig/scene-destroy/src/scene.js#L159
* ```this.fbo_texture``` https://github.com/tangrams/tangram/blob/iwillig/scene-destroy/src/scene.js#L166
* ```this.fbo_depth``` (it looks like we might be able to just make this a temp variable and not a retained property of the scene, need to look into that) https://github.com/tangrams/tangram/blob/iwillig/scene-destroy/src/scene.js#L171

I'll look for more :)