Miscellaneous fixes:

1. Update NiStencilProperty based on Shon's changes
2. Rewrote large chunks of WriteFileGroup related routines.
3. Added a WriteNifTree which allows for multiple roots (not exposed publically)
4. Added option to BindSkin to scene instead of skeleton root.
5. Fixed bug in RemoveController
6. Added helpers to NiControllerManager for sequence management
7. Propegated user_version in routines which take a version