Fix ArrowHelper so it can point downwards (+ some optimization)

While debugging normals during geometry creation I noticed that ArrowHelper wasn't able to point downwards (0,-1,0).  This PR fixes this issue and also optimizes ArrowHelper based on a suggestions from @WestLangley here: https://github.com/mrdoob/three.js/pull/2845#issuecomment-11725063

Tested lights in /editor/index.html and the /examples that referenced ArrowHelper directly.  Seems to all works.