- modified bounding box/sphere calculation to use THREE.Box3
- fixed issue with hard to specify filtering to delete
objects in the clearScene() method:
 * now all internal (custom element wrapped) items have
a "peristent" flag added to them , so that the clearScene method
ignores them
 * added ability to set persistent flag manually when using the addToScene method
 * fixes usco/usco-ultiviewer#30

- more refinements to addToScene() method
 * added updateable flag in options of addToScene method (to add added object
to the list of updateables)
 * some extra cleanup in centering of objects
 * added some helpfull docstrings