Colormaps

This implements colormaps as per https://github.com/vispy/vispy/issues/414 . It is a first pass that uses OpenGL functions. Visuals will need to integrate this, but it has already been done for the Line visual. It includes a new example 'colored_line.py' that cycles through all the colormaps.
I dunno.. we will eventually need these maps defined in both GLSL and python, and personally I would rather have the GLSL definitions attached to the python definitions. (Ideally, the GLSL could be generated from the python definition)