FIX: Fix examples

Closes #524.
Closes #522.

Ready for review/merge.
It should be correct, it's an example of using the new colormaps in a visual. If a user passes in a string, it first checks to see if it is a colormap (using get_colormap). If it is not, it then tries to construct a Function object. This allows users to pass arbitrary GLSL functions for colormaps.