Add WebWorkers to Loader service

[Morgul]

We need to add WebWorkers to the Loader service, to keep from blocking the entire client as we attempt to load some of our larger models.

Googling around, it seems like much over 100 web workers can cause issues, so we might have to be careful about simply doing 1 web worker per model; it might instead make sense to have a pool of 'loader workers' that we just farm loading out to, until their done.

[Morgul]

We need to re-evaluate what's needed, now that the AssetManager is a thing.

[Morgul]

The plan is to switch to Asset Manager (#25). Babylon.js will be added we worker loading in the future, so no need for us to worry about it.