Make Model modelMatrix use Z-up convention

We apply a rotation to the glTF model behind the scenes to make it's Y-up convention match Cesium; also needed to swizzel the BoundingSphere.
Adjust Models demo after recent fov changes

Also fix typo in specs.
Merge remote-tracking branch 'origin/master' into issue1940

Conflicts:
	Specs/Scene/ModelSpec.js
Fix Model specs by tweaking zoomTo code.
Merge remote-tracking branch 'origin/master' into issue1940
Changes after review.
Update CHANGES
Tweak CHANGES.md
Use BoundingSphere.transformWithoutScale
Merge pull request #1963 from AnalyticalGraphicsInc/issue1940

Change Model orientation convention to match the rest of Cesium