Fix a few minor issues with LindenSkeleton

1) Doesn't crash when trying to load the lod 1 mesh for eyes.
2) Can decode the "packed" skinweights.
3) Has xmldoc tags for nearly everything.

Besides these changes I've added a version of avatar_skeleton.xml to the
"openmetaverse_data" folder. The file is copyright Linden Labs, and is
published by them under CCSA.