WIP: GLSL function composition

Basically adds two files: the new Function implementation and a file with examples and tests. High level users should mostly checkout the docstring of the Function class, and the examples in test_function.py.

Everything works (or should, I hope to add a test or two). I think the only remaining issue is that I am not sure yet how variables should work out. 

No need to wait for me to move this forward. If I don't like what you've done with it, I'll nag and we can re-discuss afterwards ;)

Would it be worth providing a one-line shortcut syntax for this? I suppose this is going to be something that will come very often (`frag_code['color'] = 'attribute vec3 a_color ==> varying vec3 v_color'`, or something better if possible!...)