save / load - support 2d layer node

[kfarr]

in many cases this node is not used

here is an example of it being used
https://github.3dstreet.org/mapbox.html

in this example, the mapbox satellite street layer should be saved under 2d layer node

[Algorush]

In order for the scene loading with the mapbox component to work, I added component code (aframe-mapbox-component.min.js) to the src/lib folder and changed THREE.Math to THREE.MathUtils, since THREE.Math is not supported in new versions of three.js. Should I create separate PR for this or no?

[Algorush]

I also found that the scene does not save with all the cars on one of the streets at the intersection. After fixing this issue, I will upload a PR. I think that its related with this issue: #306

[kfarr]

closed with 3DStreet/3dstreet-editor#223