[wip] Add Hand Splay

[pehrlich]

This plugin measures how flat and stretched-out a hand is

measures how spread out and flat a hand is fires 'handSplay' and 'handUnsplay' events from the controller

Two items are made accessible through hand.data hand.data('handSplay.splay') returns the fractional splay amount, 0 to 1 hand.data('handSplay.splayed') returns a boolean: whether the hand is considered splayed or not

Currently there is an issue where flatness is measured, rather than splay. Need to use cross-product to fix.