cleanup; adding additional checks for whether on mobile and VR (otherwise don't bother polling the gamepad api)

refactoring. Also making sure that navNode won't be null before clampStep if you try to teleport to an invalid (according to the navmesh) location.

Added hack to exit scene when Oculus Go goes into standby. Clean up virtual gamepad controls on exit.

Properly show cursor when in cardboard. Note: cardboard input is broken until we upgrade to aframe 0.8.2.

use latest dists for aframe-physics-system and aframe-physics-extras (fixes #269). Fix issue where phyiscal hand attached to cursor-controller wasn't being set properly and also add back hoverable component to ducks.

add/remove interactable class instead of changing scale and handle rotation correctly with super-spawner; point to duck gltf model in assets repo (and remove scale set in the html)