Optimized renderer beta

Fixes #4628
Update loader-assimp2json example: assimp.json is the new extension for assimp2json JSON files and a format metadata header is added to the format.
Update WebGLOptimizedRenderer.js
Update WebGLOptimizedRenderer.js
Object3D: Recursively dispatch removedFromScene event. Should solve #5269.
Example clean up.
Updated builds.
Merge pull request #5275 from acgessler/dev

Update loader-assimp2json example
Merge branch 'dev' of https://github.com/AleksandrAlbert/three.js.git into dev
Merge remote-tracking branch 'upstream/dev' into dev