WIP: Volume rendering

This PR implements volume rendering. It's basically a port of the technique that I use in Visvis.

* New Volume visual with two rendering styles (iso and mip). 
* Volume example in examples/scene/volume.py Try it, and use the keys to switch cameras etc.
* Refactored cameras a bit in general
* Refactored TurnTableCamera: no more mode and distance. You can just change the fov while getting the same zoom level. Setting fov to 0 means orthographic. Zooming is done by moving camera backward.
* New FlyCamera to fly around your data. 

This is about ready. There are more things to do, but that can be picked up in another PR. (I added todo items for this in volume.py)

I wish to do more with the camera's, but we should probably discuss this first. Will make a new issue for this.

todo:

* [x] avoid warning when swithching render styles
* [x] camera's




![volume](https://cloud.githubusercontent.com/assets/1942359/5230818/8495b5ec-772c-11e4-8560-d2ac7f393b22.png)

I find the turntable interaction very strange. Right now I assume one of the corners of the volume visual gets placed at the origin, and this is the source of the problem. Maybe the center of the volume visual should be placed at the origin by default, or at least make it an option to do so.