Avatar attachment scripting bits.
Provide the option to avoid making duplicate attachments.
Update billboard when attachments change.
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 #2820 from ey6es/master

Scripting interface for avatar attachments.
Save attachment data for attachment model/skeleton model combinations, let
scripts use those saved parameters so that you can tweak the location of
(for example) the gun and have it saved appropriately.
Merge branch 'master' of https://github.com/highfidelity/hifi
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
Merge branch 'master' of https://github.com/worklist/hifi into 19507