added PerspectiveCamera.zoomTo()

I've done this a while ago and thought it might be useful to have this in three.js itself.
This function moves the camera so that it fits the bounding box of the given object.

Here's a demo: http://potree.org/stuff/threejs/camera_zoomto/examples/webgl_camera_zoomto.html
You are applying `factor` twice, so essentially, you have a `factor ^ 2` term. I am not following the math here.

What are you trying to achieve?