Arrow perspectivising proposal for issue #46

Please tell me if you like it
![basic_perp](https://f.cloud.github.com/assets/6823298/2441797/65f143d6-ae19-11e3-9cdf-a7bd4e5ba481.png)
Concerning arrow rotation subjecting to z-velocity: where can I find the code that gives an arrow initial speed or degenerates it? Because from the first glance it appears that z-velocity is always negative.