Merge viewerEntityMixin into Viewer

Deprecated `viewerEntityMixin` and moved it's functionality into `Viewer.
Cached glTF JSON
Added releaseGltfJSON
Updated CHANGES.md
Renamed releaseGltfJson
Use new cacheKey for animation cache
Added renderer resource caching to Model
Updated CHANGES.md
Added Sergio to contributors
Made releaseGltfJson and cacheKey private
Added tests for releaseGltfJson
Added unit tests for fromGltf caching
Move uniformMaps
Comment tweaks
More comment tweaks
Merge remote-tracking branch 'origin/master' into model-cache
Clean up Viewer code after merging in viewerEntityMixin.
Remove deprecated calls to viewerEntityMixin.
Merge remote-tracking branch 'origin/master' into merge-viewerEntityMixin
Start of uniform optimizations