Implement ModelMeshCollection.Enumerator and make the comments proper /// ones.

Fixes #2360
Whitespace and license this file
Make ModelBoneCollection match ModelMeshCollection, with the custom enumerator also.
Whitespace it ModelBoneCollection
Remove brackets to match ModelBoneCollection
Merge pull request #2395 from danzel/fix-1611

Give ModelMeshCollection/ModelBoneCollection custom Iterators