Added intersectsBox method to Frustum

I find it's sometimes better to test frustum culling against an axis aligned bouding box instead of a bouding sphere (for tall objects, for example), so I added a function for that. It's about half as efficient, but the gain in culling precision might be worth it, and the user might want to use that instead.

I also think intersectsObject should use boundingBox automatically if it is present. I didn't do anything in that method, but it might be something to consider.