Add lift for walking up stairs

Also some fixes for freeing avatars trapped in voxel collisions
merge upstream/master into andrew/inertia
Merge pull request #2821 from AndrewMeadows/inertia

avatar walks up ledges
Merge pull request #2822 from ey6es/master

Save attachment data for skeleton/attachment combinations so that they persist even when the attachment isn't present, and users can tweak script-attached models.
finish air guitar, gun initial position on hand tweak
guitar is now auto-attached
double fisted gun action
Merge pull request #2823 from PhilipRosedale/master

Finishing airGuitar, added autoloading guitar model
cleanup - remove unused leap fingers
Merge pull request #2824 from AndrewMeadows/inertia

cleanup - remove unused leap fingers