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
What does this quantity represent? Are you assuming `aspect > 1`?