Improve and clean examples. Update contributor list.

In #349 Eric asked to fix up glsl-sandbox-cube.py and split-screen.py. One thing led to another and I did some more:

* I scrolled to all merged PR's and update the contributors list accordingly. Quite a few people have already helped out!
* Removed split-screen.py because it tried to show something that we can now do with ViewBox
* Removed client_buffers.py because we don't support client buffers :)
* Updated animate_image_slices.py to be more interesting, and fixed a bug in 3D texture.
* Modified some demos to fix them up or make a bit more interesting.

I will probably fool with the example some more. But this PR is ready to go. This also addresses issue #405.