Allow vertical axis particle rotation constraint

![minetestnotverticalrain](https://f.cloud.github.com/assets/1996449/410695/f21a7f6e-ab75-11e2-9157-5a93cb4ac6d5.png)
No one likes rain falling sideways.
![minetestverticalrain](https://f.cloud.github.com/assets/1996449/410644/96a2d448-ab74-11e2-81e8-4a1c459f493a.png)