Normalized ray + fix crossVectors

Looks like some guys really want a normalized ray in raycaster :D #3655
I also added a ray-triangle intersection function, it looks like the same performance as using ray-plane => triangleContainsPoint. Anyway, Webgl/Canvas_interactive_cubes work fine.
Hmm I'll let you be the judge on this PR.... (btw I didn't find any examples with interactive bufferGeometry...)

However, there is a small fix for the ``crossVectors`` function.
Here's the buggy case scenario, you've got two ``Vector3`` : ``a`` and ``b``.
You want to put in ``a`` the value of 'b cross a' without creating a new object...so you do ``a.crossVectors(b,a )`` and pouf!